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TWENTIETH   CENTURY  TEXT-BOOKS 

EDITED    BY 

A.  F.  NIGHTINGALE,  Ph.D.,  LL.D. 

SUPERINTENDENT   OF  SCHOOLS,    COOK   COUNTY,    ILLINOIS 


TEXT-BOOKS   IN    BOTANY 

By  John  M.  Coulter.  Ph.D. 

HEAD  OF  DEPARTMENT  OF  BOTANY  IN  THE  UNIVERSITY 
OF   CHICAGO 


Text-Book  of  Botany.  12mo.  Illustrated. 
Cloth $1.25 

Plant  Studies.  An  Elementary  Botany.  12mo. 
Cloth $1.25 

Plant  Relations.  A  First  Book  of  Botany. 
12mo.     Cloth $1.10 

Plant  Structures.  A  Second  Book  of  Bot- 
any.    12mo.     Cloth     ....:.     $1.20 

Plants.  The  two  foregoing  in  one  volume. 
For  Normal  Schools  and  Colleges.  12mo. 
Cloth $1.80 

In  the  Twentieth  Century  Series  of  Text-Books 


D.  Appleton  and  Company,  Ne-w  York 


TWENTIETH   CENTURY   TEXT-BOOKS 


PLANT  STUDIES 

AN    ELEMENTARY   BOTANY 


BY 


JOHN   M.   COULTER,   A.M.,    Ph.D. 

HEAD   OF    DEPARTMENT   OF    BOTANY 
UNIVERSITY    OF    CHICAGO 


REVISED   EDITION 


NEW    YORK 
D.    APPLETON    AND    COMPANY 

IQII 


Copyright,  1900,  1905, 
By  D.  APPLETON  AND  C03IPANV. 


PREFACE 

This  book  has  been  prepared  in  response  to  the  earnest 
solicitation  of  those  schools  in  which  there  is  not  a  suffi- 
cient allotment  of  time  to  permit  the  development  of  plant 
ecology  and  morphology,  as  outlined  in  Plant  Relations  and 
Plant  Structures  ;  and  yet  which  are  desirous  of  imparting 
instruction  from  both  points  of  view.  To  meet  this  need, 
the  essential  portions  of  the  two  books  referred  to  have 
been  selected  and  combined,  which,  with  the  addition  of 
some  new  matter  to  give  it  logical  continuity  and  a  degree 
of  completeness,  have  been  organized  into  this  volume 
under  the  title  of  Plant  Studies. 

The  book  falls  naturally  into  two  divisions,  the  first 
fourteen  chapters  being  dominated  by  Ecology,  and  repre- 
senting the  view  point  of  Plant  Relations.  The  remaining 
eleven  chapters  are  dominated  by  Morphology,  and  present 
in  much  simpler  form,  especially  in  the  higher  groups,  the 
ideas  of  Plant  Structures.  AVhile  the  author  believes  that 
these  two  regions  of  the  book  are  put  in  proper  sequence 
for  elementary  instruction,  he  is  very  far  from  seeking  to 
impose  such  an  opinion  upon  teachers,  who  must  use  a 
sequence  adapted  to  their  own  convictions  and  material. 
Hence  many  may  prefer  to  begin  with  Chapter  XV,  and  re- 
turn to  the  preceding  chapters  later ;  or,  what  is  perhaps 


J^H- 


yi  PLANT   STUDIES 

better,  they  may  prefer  to  combine  the  two  divisions  of  the 
book  much  more  intimately. 

In  any  event,  the  book  is  not  a  laboratory  guide,  or  a 
book  merely  for  recitation,  but  is  for  reading  and  study  in 
connection  with  laboratory  and  field-work.  The  intention 
is  to  present  a  connected,  readable  account  of  some  of  the 
fundamental  facts  of  botany,  and  to  give  a  certain  amount 
of  information.  If  it  performs  no  other  service  in  the 
schools,  however,  its  purpose  will  be  defeated.  It  is  entire- 
ly too  compact  for  any  such  use,  for  great  subjects,  which 
should  involve  a  large  amount  of  observation,  are  often 
merely  suggested. 

It  is  intended  to  serve  as  a  supplement  to  three  far  more 
important  factors  :  (1)  the  teacher^  who  must  amplify  and 
suggest  at  every  point ;  (2)  the  laboratory^  which  must  bring 
the  pupil  face  to  face  with  plants  and  their  structures; 
(3)  field-imrh^  which  must  relate  the  facts  observed  in  the 
laboratory  to  their  actual  place  in  Nature,  and  must  bring 
new  facts  to  notice  which  can  be  observed  nowhere  else. 
Taking  the  results  obtained  from  these  three  factors,  the 
book  seeks  to  organize  them,  and  to  suggest  explanations. 
It  seeks  to  do  this  in  two  ways :  (1)  by  means  of  the  text, 
which  is  intended  to  be  clear  and  untechnical,  but  compact; 
(2)  by  means  of  the  illustrations,  which  must  be  studied  as 
carefully  as  the  text,  as  they  are  only  second  in  importance 
to  the  actual  material.  Especially  is  this  true  in  reference 
to  the  landscapes,  many  of  which  can  not  be  made  a  part  of 
experience. 

My  thanks  are  due  to  various  members  of  the  Depart- 
ment of  Botany  of  the  university  for  preparing  and  select- 
ing illustrations.     The  illustrations  of  the  first  fourteen 


PREFACE  vii 

chapters  were  under  the  general  direction  of  Dr.  Henry  C. 
Cowles,  while  those  of  the  remaining  chapters  were  pro- 
vided by  Dr.  Otis  W.  Caldwell.  In  this  work  Dr.  Caldwell 
had  the  very  efficient  assistance  of  S.  M.  Coulter,  B.  A.  Gold- 
berger,  J.  G.  Land,  and  A.  0.  Moore,  whose  names  appear 
in  connection  with  the  drawings  they  furnished.  Grateful 
acknowledgment  should  also  be  made  to  Dr.  W.  J.  Beal, 
whose  little  book  entitled  Seed  Dispersal  furnished  several 
illustrations ;  and  to  Professor  George  F.  Atkinson,  whose 
excellently  illustrated  Elementary  Botany  performed  a  like 
service.  Both  of  these  authors  are  credited  in  connection 
with  the  illustrations  used  from  their  works.  The  fine 
illustrations  from  Kerner  and  from  Schimper,  and  from 
other  authors,  will  also  be  recognized ;  but  their  names  will 
all  be  found  in  the  legends. 

John  M.  Coultee. 
The  University  of  Chicago,  June,  1900. 


PKEFACE   TO   THE  EEVISED   EDITION 

During  the  last  four  years  the  science  of  Botany  has 
made  rapid  progress,  both  in  the  addition  of  new  facts 
and  in  changed  points  of  view.  Some  of  this  progress 
affects  Plant  Studies,  and  it  is  recorded  in  this  revised 
edition  so  far  as  it  can  be  without  a  complete  rewriting 
of  the  volume.  Changes  will  be  found,  therefore,  in  state- 
ments of  fact,  in  points  of  view,  in  terminology,  in  illus- 
trations,  and  also  in  the  addition  of  new  material. 

John  M.  Coulter. 

The  University  of  Chicago,  April,  1904. 


CONTENTS 

CHAPTER  PAOS 

I.— Introduction 1 

II. — Foliage  leaves:  The  light  relation  ....  6 

III. — Foliage  leaves  :  Function,  structure,  and  protection  28 

IV.— Shoots 53 

V. — Roots 89 

VI. — Reproductive  organs 109 

VII. — Flowers  and  insects 123 

VIII. — An  individual  plant  in  all  of  its  relations     .        .  138 

IX. — The  struggle  for  existence 142 

X. — The  nutrition  of  plants 149 

XL— Plant  associations:  Ecological  factors     .        ,       .169 

XII. — Hydrophyte  associations 177 

XIII. — Xerophyte  associations 188 

XIV. — Mesophyte  associations 214 

XV. — The  plant  groups 221 

XVI. — Thallophytes  :   Alg^ 224 

XVII. — The  great  groups  of  Alg^ 232 

XVIII. — Thallophytes:  Fungi 264 

XIX. — Bryophytes  (moss  plants) 299 

XX. — The  great  groups  of  Bryophytes         ....  308 

XXI. — Pteridophytes  (fern  plants) 320 

XXII. — The  great  groups  of  Pteridophytes    ....  334 

XXIII. — Spermatophytes  :  Gymnosperms 343 

XXIV. — Spermatophytes:  Angiosperms 358 

XXV. — Monocotyledons  and  Dicotyledons       ....  376 

Glossary 383 

Index 389 


BOTANY 

PLANT    STUDIES 


CHAPTEE  I 

INTRODUCTION 

1.  General  relations. — Plants  form  the  natural  covering 

of  the  earth's  surface.  So  generally  is  this  true  that  a  land 
surface  without  plants  seems  remarkable.  Not  only  do 
plants  cover  the  land,  but  they  abound  in  waters  as  well, 
both  fresh  and  salt  waters.  They  are  wonderfully  varied  in 
Bize,  ranging  from  huge  trees  to  forms  so  minute  that  the 
microscope  must  be  used  to  discover  them.  They  are  also 
exceedingly  variable  in  form,  as  may  be  seen  by  comparing 
trees,  lilies,  ferns,  mosses,  mushrooms,  lichens,  and  the 
green  tliready  growths  {algce)  found  in  water. 

2.  Plant  associations.— One  of  the  most  noticeable  facts 
in  reference  to  plants  is  that  they  do  not  form  a  monot- 
onous covering  for  the  earth's  surface,  but  that  there  are 
forests  in  one  place,  thickets  in  another,  meadows  in 
another,  swamp  growths  in  another,  etc.  In  this  way  the 
general  appearance  of  vegetation  is  exceedingly  varied, 
and  each  appearance  tells  of  certain  conditions  of  living. 
These  groups  of  plants  living  together  in  similar  conditions, 
as  trees  and  other  plants  in  a  forest,  or  grasses  and  other 
plants  in  a  meadow,  are  known  as ^lant  associations.    These 

1 


— ^-     ^'•opKirrvoF 


8  PLANT  STUDIES 

associations  are  as  numerous  as  are  the  conditions  of  living, 
and  it  may  be  said  that  each  association  has  its  own  special 
regulations,  which  admit  certain  plants  and  exclude  others. 
The  study  of  plant  associations  to  determine  their  conditions 
of  living  is  one  of  the  chief  purposes  of  botanical  field  work. 

3.  Plants  as  living  things. — Before  engaging  in  a  study 
of  associations,  however,  one  must  discover  in  a  general  way 
how  the  individual  plant  lives,  for  the  plant  covering  of  the 
earth^s  surface  is  a  living  one,  and  plants  must  always  be 
thought  of  as  living  and  at  work.  They  are  as  much  alive 
as  are  animals,  and  so  far  as  mere  living  is  concerned  they 
live  in  much  the  same  way.  Nor  must  it  be  supposed  that 
animals  move  and  plants  do  not,  for  while  more  animals  than 
plants  have  the  power  of  moving  from  place  to  place,  some 
plants  have  this  power,  and  those  that  do  not  can  move  cer- 
tain parts.  The  more  we  know  of  living  things  the  more  is 
it  evident  that  life  processes  are  alike  in  them  all,  whether 
plants  or  animals.  In  fact,  there  are  some  living  things 
about  which  we  are  uncertain  whether  to  regard  them  as 
plants  or  animals. 

4.  The  plant  body. — Every  plant  has  a  body,  which  may 
be  alike  throughout  or  may  be  made  up  of  a  number  of 
different  parts.  When  the  green  thready  plants  {algm),  so 
common  in  fresh  water,  are  examined,  the  body  looks  like 
a  simple  thread,  without  any  special  parts  ;  but  the  body  of 
a  lily  is  made  up  of  such  dissimilar  parts  as  root,  stem, 
leaf,  and  flower  (see  Figs.  75,  144,  161,  169).  The  plant 
without  these  special  parts  is  said  to  be  simple,  the  plant 
with  them  is  called  complex.  The  simple  plant  lives  in 
the  same  way  and  does  the  same  kind  of  work,  so  far  as 
living  is  concerned,  as  does  the  complex  plant.  The  differ- 
ence is  that  in  the  case  of  the  simple  plant  its  whole  body 
does  every  kind  of  work  ;  while  in  the  complex  plant 
different  kinds  of  work  are  done  by  different  regions  of  the 
body,  and  these  regions  come  to  look  unlike  when  differ- 
ent shapes  are  better  suited  to  different  work,  as  in  the 


INTRODUCTION  8 

case  of  a  leaf  and  a  root,  two  regions  of  the  body  doing 
different  kinds  of  work. 

5.  Plant  organs. — These  regions  of  the  plant  body  thus 
set  apart  for  special  purposes  are  called  organs.  The  sim- 
plest of  plants,  therefore,  do  not  have  distinct  organs, 
while  the  complex  plants  may  have  several  kinds  of  organs. 
All  plants  are  not  either  very  simple  or  very  complex,  but 
beginning  with  the  simplest  plants  one  may  pass  to  others 
not  quite  so  simple,  then  to  others  more  complex,  and  so 
on  gradually  until  the  most  complex  forms  are  reached. 
This  process  of  becoming  more  and  more  complex  is  known 
as  differentiation,  which  simply  means  the  setting  apart  of 
different  regions  of  the  body  to  do  different  kinds  of  work. 
The  advantage  of  this  to  the  plant  becomes  plain  by  using 
the  common  illustration  of  the  difference  between  a  tribe 
of  savages  and  a  civilized  community.  The  savages  all  do 
the  same  things,  and  each  savage  does  everything.  In  the 
civilized  community  some  of  the  members  are  farmers, 
otliers  bakers,  others  tailors,  others  butchers,  etc.  This  is 
what  is  known  as  ^'  division  of  labor,"  and  one  great  advan- 
tage it  has  is  that  every  kind  of  work  is  better  done.  Dif- 
ferentiation of  organs  in  a  plant  means  to  the  plant  just 
what  division  of  labor  means  to  the  community ;  it  results 
in  more  work,  and  better  work,  and  new  kinds  of  work. 
The  very  simple  plant  resembles  the  savage  tribe,  the  com- 
plex plant  resembles  the  civilized  community.  It  must  be 
understood,  however,  that  in  the  case  of  plants  the  differ- 
entiation referred  to  is  one  of  organs  and  not  of  individuals. 

6.  Plant  functions. — Whether  plants  have  many  organs, 
or  few  organs,  or  no  organs,  it  should  be  remembered  that 
they  are  all  at  work,  and  are  all  doing  the  same  essential 
things.  Although  many  different  kinds  of  work  are  being 
carried  on  by  plants,  they  may  all  be  put  under  two  heads, 
nutrition  and  reproduction.  Every  plant,  whether  simple 
or  complex,  must  care  for  two  things  :  (1)  its  own  support 
(nutrition),   and  (2)   the  production  of   other  plants  like 


4  PLAJST   STUDIES 

itself  (reproduction).  To  the  great  work  of  nutrition  many 
kinds  of  work  contribute,  and  the  same  is  true  of  repro- 
duction. Nutrition  and  reproduction,  however,  are  the 
two  primary  kinds  of  work,  and  it  is  interesting  to  note 
that  the  first  advance  in  the  differentiation  of  a  simple 
plant  body  is  to  separate  the  nutritive  and  reproductive 
regions.  In  tlie  complex  plants  there  are  nutritive  organs 
and  reproductive  organs  ;  by  which  is  meant  that  there  are 
distinct  organs  which  specially  contribute  to  the  work  of 
nutrition,  and  others  which  are  specially  concerned  with 
the  work  of  reproduction.  The  different  kinds  of  work  are 
conveniently  spoken  of  ^s  functions,  each  organ  having  one 
or  more  functions. 

7.  Life-relations. — In  its  nutritive  and  reproductive  work 
the  plant  is  very  dependent  upon  its  surroundings.  It 
must  receive  material  from  the  outside  and  get  rid  of  waste 
material ;  and  it  must  leave  its  offspring  in  as  favorable 
conditions  for  living  as  possible.  As  a  consequence,  every 
organ  holds  a  definite  relation  to  something  outside  of  it- 
self, known  as  its  life-relation.  For  example,  green  leaves 
are  definitely  related  to  light,  many  roots  are  related  to 
soil,  certain  jilants  are  related  to  abundant  water,  some 
plants  are  related  to  other  plants  or  animals  (living  as 
parasites),  etc.  A  plant  with  several  organs,  therefore, 
may  hold  a  great  variety  of  life-relations,  and  it  is  quite  a 
complex  problem  for  such  a  plant  to  adjust  all  of  its  parts 
properly  to  their  necessary  relations.  The  study  of  the 
life-relations  of  plants  is  a  division  of  Botany  known  as 
Ecology,  and  presents  to  us  many  of  the  most  important 
problems  of  plant  life. 

It  must  not  be  supposed  that  any  plant  or  organ  holds 
a  perfectly  simple  life-relation,  for  it  is  affected  by  a  great 
variety  of  things.  A  root,  for  instance,  is  affected  by  light, 
gravity,  moisture,  soil  material,  contact,  etc.  Every  or- 
gan, therefore,  must  adjust  itself  to  a  very  complex  set  of 
life-relations,  and  a  plant  with  several  organs  has  so  many 


INTRODUCTION 


delicate  adjustments  to  care  for  that  it  is  really  impossi- 
ble, as  yet,  for  us  to  explain  why  all  of  its  parts  are  placed 
just  as  they  are.  In  the  beginning  of  the  study  of  plants, 
only  some  of  the  most  prominent  functions  and  life-rela- 
tions can  be  considered.  In  order  to  do  this,  it  seems  bet- 
ter to  begin  with  single  organs,  and  afterwards  these  can 
be  put  together  in  the  construction  of  the  whole  plant. 


CHAPTER  II 

FOLIAGE  LEAVES:   THE  LIGHT-RELATION 

8.  Definition. — A  foliage  leaf  is  the  ordinary  green  leaf, 
and  is  a  very  important  organ  in  connection  with  the  work 
of  nutrition.  It  must  not  be  thought  that  the  work  done  by 
such  a  leaf  cannot  be  done  by  green  plants  which  have  no 
leaves,  as  the  algae,  for  example.  A  leaf  is  simply  an  or- 
gan set  apart  to  do  such  work  better.  In  studying  the 
work  of  a  leaf,  therefore,  we  have  certain  kinds  of  work 
set  apart  more  distinctly  than  if  they  were  confused  with 
other  kinds.  For  this  reason  the  leaf  is  selected  as  an  in- 
troduction to  some  of  the  important  work  carried  on  by 
plants,  but  it  must  not  be  forgotten  that  a  plant  does  not 
need  leaves  to  do  this  work  ;  they  simply  enable  it  to  work 
more  effectively. 

9.  Position. — It  is  easily  observed  that  foliage  leaves 
grow  only  upon  stems,  and  that  the  stems  which  bear  them 
always  expose  them  to  light ;  that  is,  such  leaves  are  aerial 
rather  than  subterranean  (see  Figs.  1,  75,  174).  Many 
stems  grow  underground,  and  such  stems  either  bear  no 
foliage  leaves,  or  are  so  placed  that  the  foliage  leaves  are 
sent  above  the  surface,  as  in  most  ferns  and  many  plants  of 
the  early  spring  (see  Figs.  45,  46,  144). 

10.  Color. — Another  fact  to  be  observed  is  that  foliage 
leaves  have  a  characteristic  green  color,  a  color  so  universal 
that  it  has  come  to  be  associated  with  plants,  and  espe- 
cially with  leaves.  It  is  also  evident  that  this  green  color 
holds  some  necessary  relation  to  light,  for  the  leaves  of 

plan^  grown  in  the  dark,  as  potatoes  sprouting  in  a  cellar, 
6 


FOLIAG.     LEAVES:    THE   LIGHT-KELATION  7 

do  not  develop  this  color.  Even  when  leaves  have  devel- 
oped the  green  color  they  lose  it  if  deprived  of  light,  as  is 
shown  by  the  process  of  blanching  celery,  and  by  the  effect 
on  the  color  of  grass  if  a  board  has  lain  upon  it  for 
some  time.  It  seems  plain,  therefore,  that  the  green  color 
found  in  working  foliage  leaves  depends  upon  light  for  its 
existence. 

We  conclude  that  at  least  one  of  the  essential  life-rela- 
tions of  a  foliage  leaf  is  what  may  be  called  the  liglit-r ela- 
tion. This  seems  to  explain  satisfactorily  why  such  leaves 
are  not  developed  in  a  subterranean  position,  as  are  many 
stems  and  most  roots,  and  why  plants  which  produce  them 
do  not  grow  in  the  dark,  as  in  caverns.  The  same  green, 
and  hence  the  same  light-relation,  is  observed  in  other 
parts  of  the  plant  as  well,  and  in  plants  without  leaves,  the 
only  difference  being  that  leaves  display  it  most  conspicu- 
ously. Another  indication  that  the  green  color  is  con- 
nected with  light  may  be  obtained  from  the  fact  that  it  is 
found  only  in  the  surface  region  of  plants.  If  one  cuts 
across  a  living  twig  or  into  a  cactus  body,  the  green  color 
will  be  seen  only  in  the  outer  part  of  the  section.  The  con- 
clusion is  that  the  leaf  is  a  special  organ  for  the  light-re- 
lation. Plants  sometimes  grow  in  such  situations  that  it 
would  be  unsafe  for  them  to  display  leaves,  or  at  least  large 
leaves.  In  such  a  case  the  work  of  the  leaves  can  be  thrown 
upon  the  stem.  A  notable  illustration  of  this  is  the  cactus 
plant,  which  produces  no  foliage  leaves,  but  whose  stem  dis- 
plays the  leaf  color. 

11.  An  expanded  organ. — Another  general  fact  in  refer- 
ence to  the  foliage  leaf  is  that  in  most  cases  it  is  an  expanded 
organ.  This  means  that  it  has  a  great  amount  of  surface 
exposed  in  comparison  with  its  mass.  As  this  form  is  of 
such  common  occurrence  it  is  safe  to  conclude  that  it  is  in 
some  way  related  to  the  work  of  the  leaf,  and  that  whatever 
work  the  leaf  does  demands  an  exposure  of  surface  rather 
than  thickness  of  body.  It  is  but  another  step  to  say  that 
2 


« 


PLANT   STUDIES 


the  amount  of  work  an  active  leaf  can  do   vvill  depend  in 
part  upon  the  amount  of  surface  it  exposes. 


THE    LIGHT-RELATION 


12.  The  general  relation. — The  ordinary  position  of  the 
foliage  leaf  is  more  or  less  horizontal.  This  enables  it  to 
receive  the  direct  rays  of  light  ujion  its  upper  surface.     In 

this  way  more  rays  of 
light  strike  the  leaf  sur- 
face than  if  it  stood  ob- 
liquely or  on  edge.  It  is 
often  said  that  leaf  blades 
are  so  directed  that  the 
flat  surface  is  at  right 
angles  to  the  incident 
rays  of  light.  While  this 
may  be  true  of  horizon- 
tal leaves  in  a  general 
way,  the  observation  of 
almost  any  plant  will 
show  that  it  is  a  very 
general  statement,  to 
which  there  are  numerous 
exceptions  (see  Fig.  1). 
Leaves  must  be  arranged 
to  receive  as  much  light 
as  possible  to  help  in 
their  work,  but  too  much 
light  will  destroy  the 
green  substance  {chloro- 
pliyll),  which  is  essential 
to  the  work.  The  adjust- 
ment to  light,  therefore, 
is  a  delicate  one,  for 
there  must  be  just  enough 


Fig.  1,  The  leaves  of  this  plant  (Ficus)  are 
in  general  horizontal,  but  it  will  be  seen 
that  the  lower  ones  are  directed  down- 
ward, and  that  the  leaves  become  more 
horizontal  as  the  stem  is  ascended.  It 
will  also  be  seen  that  the  leaves  are  so 
broad  that  there  are  few  vertical  rows. 


FOLIAGE   LEAVES:    THE   LIGHT-KELATION  9 

and  not  too  much.  The  danger  from  too  much  light  is 
not  the  same  in  the  case  of  all  leaves,  even  on  the  same 
plant,  for  some  are  more  shaded  than  others.  Leaves  also 
have  a  way  of  protecting  themselves  from  too  intense  light 
by  their  structure,  rather  than  by  a  cliange  in  their  posi- 
tion. It  is  evident,  therefore,  that  the  exact  position  which 
any  particular  leaf  holds  in  relation  to  light  depends  upon 
many  circumstances,  and  cannot  be  covered  by  a  general 
rule,  except  that  it  seeks  to  get  all  tlie  light  it  can  without 
danger. 

13.  Fixed  position. — Leaves  differ  very  much  in  the  power 
of  adjusting  their  position  to  the  direction  of  the  light. 


Fig.  2.    The  day  and  night  positions  of  the  leaves  of  a  member  {A7}iicia)  of  the  pea 
family.— After  Strasburger. 

Most  leaves  when  fully  grown  are  in  a  fixed  position  and 
cannot  change  it,  however  unfavorable  it  may  prove  to  be, 
except  as  they  are  blown  about.  Such  leaves  are  said  to 
hs,\ejixed  liglit  positions.  This  position  is  determined  by 
the  light  conditions  that  prevailed  while  the  leaf  was  grow- 
ing and  able  to  adjust  itself.  If  these  conditions  continue, 
the  resulting  fixed  position  represents  the  best  one  that  can 
be  secured  under  the  circumstances.  The  leaf  may  not 
receive  the  rays  of  liglit  directly  throughout  the  whole 
period  of  daylight,  but  its  fixed  position  is  such  that  it 
probably  receives  more  light  tlian  it  would  in  any  other 
position  that  it  could  secure. 


10 


PLANT  STCDIES 


1-4.  Motile  leaves. — There  are  leaves,  however,  which 
have  no  fixed  light  ^^osition,  but  are  so  constructed  that 
they  can  shift  their  position  as  the  direction  of  the  light 
changes.     Such  leaves  are  not  in  the  same  position  in  the 

afternoon  as  in  tlie 
forenoon,  and  their 
night  position  may  be 
very  different  from 
either  (see  Figs.  2,  3a, 
3 J,  4).  Some  of  the 
common  house  plants 
show  this  power.  In 
the  case  of  the  com- 
mon Oxalis  the  night 
mu  ^         -.-      ,.u  ,  *    jt-  ^       position  of  the  leaves 

The  day  position  of  the  leaves  of  redbud         | 

{Cercis).-Mtex  Arthur.  is  remarkably  different 


Fig.  3a. 


from  the  position  in  light. 
If  such  a  plant  is  exposed 
to  the  light  in  a  window  and 
the  positions  of  the  leaves 
noted,  and  then  turned 
half  way  around,  so  as  to 
bring  the  other  side  to  the 
light,  the  leaves  may  be 
observed  to  adjust  them- 
selves gradually  to  the 
changed  light-relations. 

15.    Compass    plants. — A 
striking    illustration    of    a 

special  light  position  is  found  in  the  so-called  *^' compass 
plants."  The  best  known  of  these  plants  is  the  rosin-weed 
of  the  prairie  region.  Growing  in  situations  exposed  to 
intense  light,  the  leaves  are  turned  edgewise,  the  flat  faces 
being  turned  away  from  the  intense  rays  of  midday,  and 
directed  towards  the  rays  of  less  intensity  ;  that  is,  those  of 


Fig.  36.    The  night  position  of  the  leaves 
of  redbud  {Cercis).—Micv  Arthur. 


FOLIAGE   LEAVES:    THE   LIGHT-RELATION 


11 


Fig.  4.  Two  t;ensitive  plants,  showiiiLC  tlu'  n 
leaves  and  numerous  leaflets  expandt-d 
folded  together  and  the  leaves  drooping.- 


U'avcs.  TIk'  iilaut  lo  the  left  has  its 
the  one  to  tiie  right  shows  the  leaflets 
After  Kerner. 


the  morning  and  evening  (see  Fig.  170).  As  a  result,  the 
plane  of  the  leaf  lies  in  a  general  north  and  south  direc- 
tion. It  is  a  significant  fact  that  when  the  plant  grows  in 
shaded  places  the  leaves  do  not  assume  any  such  position. 
It  seems  evident,  therefore,  that  the  position  has  something 

It 


to  do  with  avoiding  the  danger  of  too  intense 


iight. 


12 


PLANT  STUDIES 


Fig.  5.  The  common  prickly  lettuce  {Lactuca 
Scariola),  showing  the  leaves  standing  edge- 
wise, and  in  a  general  north  and  south  plane. 
— After  Akthur  and  MacDougal. 


must  not  be  supposed 
that  there  is  any  ac- 
curacy in  the  north  or 
south  direction,  as  the 
edgewise  position 
seems  to  be  the  signifi- 
cant one.  In  the  ros- 
in-weed probably  the 
north  and  south  direc- 
tion is  the  prevailing 
one ;  but  in  the  prickly 
lettuce,  a  very  common 
weed  of  waste  grounds, 
and  one  of  the  most 
striking  of  the  compass 
plants,  the  edgewise 
position  is  frequently 
assumed  without  any 
special  reference  to  the 
north  or  south  direc- 
tion of  the  apex  (see 
Fig.  5). 

IG.  Heliotropism. — 
The  property  of  leaves 
and  of  other  organs 
of  responding  to  light 
is  known  as  heliotro- 
pism, light  being  one 
of  the  most  important 
of  those  external  influ- 
ences to  which  plant 
organs  respond  (see 
Figs.  6,  43). 

It  should  be  under- 
stood clearly  that  this 
is  but  a  slight  glimpse 


FOLIAGE    LEAVES:    THE    LIGHT-RELATION 


13 


Fig.  6.    These  plants  are  growing  near  a  window.    It  will  be  noticed  that  the  stems 
bend  strongly  towards  the  light,  and  that  the  leaves  face  the  light. 

of  tiie  most  obvious  relations  of  foliage  leaves  to  liglit^  and 
that  the  important  part  which  heliotropism  plays,  not  only 
in  connection  with  foliage  leaves,  but  also  in  connection 
with  other  plant  organs,  is  one  of  the  most  important  and 
extensive  subjects  of  plant  physiology. 


RELATIOi^^    OF    LEAVES   TO    OXE    ANOTHER 

A.    0)1  erect  stems 

In  view  of  what  has  been  said,  it  would  seem  that  the 
position  of  foliage  leaves  on  the  stem,  and  their  relation  to 
one  another,  must  be  determined  to  some  extent  by  the 
necessity  of  a  favorable  light-relation.  It  is  apparent  that 
the  conditions  of  the  problem  are  not  the  same  for  an  erect 
as  for  a  horizontal  stem. 

17.  Relation  of  breadth  to  number  of  vertical  rows. — 
Upon  an  erect  stem  it  is  observed  that  the  leaves  are  usu- 


14 


PLANT  STUDIES 


ally  arranged  in  a  definite  number  of  vertical  rows.  It  is 
to  the  advantage  of  the  plant  for  these  leaves  to  shade  one 
another  as  little  as  possible.  Therefore,  the  narrower  the 
leaves,  the  more  numerous  may  be  the  vertical  rows  (see 

Figs.  1,  8)  ;  and 
the  broader  the 
leaves  the  fewer 
the  vertical  rows 
(see  Fig.  1).  A 
relation  exists, 
therefore,  be- 
tween the  breadth 
of  leaves  and  the 
number  of  verti- 
cal rows,  and  the 
meaning  of  this 
becomes  plain 
when  the  light-re- 
lation is  consid- 
ered. 

18.  Relation  of 
length  to  the  dis- 
tance between 
leaves  of  the  same 
row. — Tlie  leaves 
in  a  vertical  row 
may  be  close  together  or  far  apart.  If  they  should  be  close 
together  and  at  the  same  time  long,  it  is  evident  that  they 
will  shade  each  other  considerably,  as  the  light  cannot  well 
strike  in  between  them  and  reach  the  surface  of  the  lower 
leaf.  Therefore,  the  closer  together  the  leaves  of  a  verti- 
cal row,  the  shorter  are  the  leaves  ;  and  the  farther  apart 
the  leaves  of  a  row,  the  longer  may  they  be.  Short  leaves 
permit  the  light  to  strike  between  them  even  if  they  are 
close  together  on  the  stem  ;  and  long  leaves  permit  the 
same  thing  only  when  they  are  far  apart  on  the  stem.     A 


Fig.  7. 


An  Easter  lily,  shovviug  narrow  leaves  and 
numerous  vertical  rows. 


FOLIAGE   LEAVES  :    THE   LIGHT-RELATION 


15 


relation  is  to  be  observed,  therefore,  between  the  length 
of  leaves  and  their  distance  apart  in  the  same  vertical  row. 
The  same  kind  of  relation  can  be  observed  in  reference 
to  the  breadth  of  leaves,  for  if  leaves  are  not  only  short  but 
narrow  they  can  stand  very  close  together.  It  is  thus  seen 
that  the  length  and  breadth  of  leaves,  the  number  of  ver- 
tical rows  on  the  stem,  and  the  distance  between  the  leaves 


Fig.  8.    A  dragon-tree,  showing  narrow  leaves  extending  in  all  directions,  and  uumer 
ous  vertical  rows. 


of  any  row,  all  have  to  do  with  the  light-relation  and  are 
answers  to  the  problem  of  shading. 

19.  Elongation  of  the  lower  petioles. — There  is  still 
another  common  arrangement  by  wliich  an  effective  light- 
relation  is  secured  by  leaves  wliich  are  broad  and  placed 
close  together  on  the  stem.  In  such  a  case  the  stalks 
{petioles)  of  the  lower  leaves  become  longer  than  those 
above  and  thus  thrust  their  blades  beyond  the  shadow  (see 
Fig.   9).      It  may  be  noticed  that  it  is  very  common   to 


16 


PLANT  STUDIES 


find  the  lowest  leaves  of  a  plant  the  largest  and  with  the 
longest  petioles,  even  when  the  leaves  are  not  very  close 
together  on  the  stem. 

It  must  not  be  supposed  that  by  any  of  these  devices 
shading  is  absolutely  avoided.  This  is  often  impossible  and 
sometimes   undesirable.     It  simply   means  that   by  these 


Fig.  9.    A  plant  {Saintpaulia)  with  the  lower  petioles  elongated,  thiiistiug  the  blades 
beyond  the  shadow  of  the  upper  leaves.    A  loose  rosette. 


arrangements  the  most  favorable  light-relation  is  sought  by 
avoiding  too  great  shading. 

20.  Direction  of  leaves. — Not  only  is  the  position  on  the 
stem  to  be  observed,  but  the  direction  of  leaves  may  result 
in  a  favorable  relation  to  light.  It  is  a  very  common  thing 
to  find  a  plant  with  a  cluster  of  comparatively  large  leaves 
at  or  near  the  base,  where  they  are  in  no  danger  of  shading 
other  leaves,  and  with  the  stem  leaves  gradually  becoming 


FOLIAGE   LEAVES  :   THE   LIGHT-RELATION 


17 


smaller  and  less  liorizontal  toward  tlie  apex  of  the  stem 
(see  Figs.  10,  13).  The  common  shepherd's  purse  and  the 
mullein  may  be  taken  as  illustrations.  By  this  arrange- 
ment all  the  leaves  are  very 
completely  exposed  to  the 
light. 

21.  The  rosette  habit.— 
The  habit  of  producing  a 
cluster  or  rosette  of  leaves 
at  the  base  of  the  stem  is 
called  the  rosette  hahit. 
Often  this  rosette  of  leaves 
at  the  base,  frequently  lying 
flat  on  the  ground  or  on  the 
rocks,  includes  the  only  fo- 
liage leaves  the  plant  pro- 
duces. It  is  evident  that  a 
rosette,  in  which  the  leaves 
must  overlap  one  another 
more  or  less,  is  not  a  very 
favorable  light  arrange- 
ment, and  therefore  it  must 
be  that  something  is  being 
provided  for  besides  the 
light-relation  (see  Figs.  11, 
12,  13).  AVhat  this  is  will 
appear  later,   but   even   in 

this  comparatively  unfavorable  light  arrangement,  there  is 
evident  adjustment  to  secure  the  most  light  possible  undo 
the  circumstances.  The  lowest  leaves  of  the  rosette  are 
the  longest,  and  the  ujiper  (or  inner)  ones  become  gradu- 
ally shorter,  so  that  all  tlie  leaves  have  at  least  a  part 
of  the  surface  exposed  to  light.  The  overlapped  base  of 
such  leaves  is  not  expanded  as  much  as  the  exposed  apex, 
and  hence  they  are  mostly  narrowed  at  the  base  and  broad 
at  the  apex.     This  narrowing  at   the   base   is   sometimes 


Fig,  10,  A  plant  (Echeveria)  with  fleshy 
leaves,  showing  large  horizontal  ones 
at  base,  and  others  becoming  smaller 
and  more  directed  upward  as  the 
stem  is  ascended. 


18 


PLANT   STUDIES 


carried  so  far  that  most  of  the  part  which  is  covered  is 
but  a  stem  (petiole)  for  the  upper  part  (blade)  which  is 
exposed. 

In  many  plants  which  do  not  form  close  rosettes  a  gen- 


FiG.  11.  A  group  of  live-for-evers,  illustrating  the  rosette  habit  and  the  light-relation. 
In  the  rosettes  it  will  be  observed  how  the  leaves  are  fitted  together  and  diminish 
in  size  inwards,  so  that  excessive  shading  is  avoided.  The  individual  leaves  also 
become  narrower  where  they  overlap,  and  are  broadest  where  they  are  exposed  to 
light.    In  the  background  is  a  plant  showing  leaves  in  very  definite  vertical  rows. 


eral  rosette  arrangement  of  the  leaves  may  be  observed  by 
looking  down  upon  them  from  above  (see  Fig.  9),  as  in  some 
of  the  early  buttercups  which  are  so  low  that  the  large 
leaves  would  seriously  shade  one  another,  except  that  the 
lower  leaves  have  longer  petioles  than  the  upper,  and  so 
reach  beyond  the  shadow. 


FOLIAGE   LEAVES:    THE   LIGHT-RELATION 


19 


Fio.  12.  Two  clumps  of  rosettes  of  the  house  leek  (Semper%'i>-um),  the  one  to  the 
right  showing  the  compact  winter  condition,  the  one  to  the  left  with  rosettes  more 
open  after  being  kept  indoors  for  several  days. 


22.  Branched  leaves. — Another  notable  feature  of  foliage 
leaves,  which  has  something  to  do  with  the  light-relation, 
is  that  on  some  plants  the  blade  does  not  consist  of  one 
piece,  but  is  lobed  or  even  broken  up  into  separate  pieces. 
When  the  divisions  are  distinct  they  are  called  leaflets,  and 
every  gradation  in  leaves  can  be  found,  from  distinct  leaf- 
lets to  lobed  leaves,  toothed  leaves,  and  finally  those  whose 
margins  are  not  indented  at  all  (entire).  This  difference 
in  leaves  i:>robably  has 

more   important  rea-  i^'^i'l'^i 

sons  than  the  light- 
relation,  but  its  sig- 
nificance may  be  ob- 
served in  this  connec- 
tion. In  those  plants 
whose  leaves  are  un- 
divided, the  leaves 
generally  either  di- 
minish in  size  toward 
the  top   of  the  stem, 

or  the  lower    ones    de-  ^^^^^     The  leavesof  abelinower  (Ca;«;,a;m/a), 

Velop  longer    petioles.  showing  the  rosette  arrangement.    The  lower 

In    this   case    the   gen-  ff  ?'"'  ^'^  Buccessively  longer,  carrying  their 

^  blades  bevoud  the  shadow  of  the  blades  above. 

eral    outline    of     the  -After  kekneu. 


Fig.  14.  A  group  of  leaves,  showing  how  branched  leaves  overtop  each  other  without 
dangerous  shading.  It  will  be  seen  that  the  larger  blades  or  less-brauched  leaves 
are  towards  the  bottom  of  the  group. 


FOLIAGE   leaves:    THE   LIGHT-RELATION  21 

plant  is  conical,  a  form  very  common  in  herbs  with  entire 
or  nearly  entire  leaves.  In  plants  whose  leaf  blades  are 
broken  up  into  leaflets  {compound  or  branched  leaves), 
however,  no,  such  diminution  in  size  toward  the  top  of  the 
stem  is  necessary  (see  Fig.  17),  though  it  may  frequently 


Fig.  15.    A  plant  allowing  much-branched  leaves,  which  occur  in  great  profusion  with- 
out cutting  off  the  light  from  one  another. 


occur.  When  a  broad  blade  is  broken  up  into  leaflets 
the  danger  of  shading  is  very  much  less,  as  the  light  can 
strike  through  between  the  upper  leaflets  and  reach  the 
leaflets  below.  On  the  lower  leaves  there  will  be  splotches 
of  light  and  shadow,  but  they  will  shift  throughout  the 
day,  so  that  probably  a  large  part  of  the  leaf  will  receive 
light  at  some  time   during  the  day   (see  Fig.   14).     The 


22 


PLANT   STUDIES 


general  outline  of  such  a  plant,  therefore,  is  usually  not 
conical,  as  in  the  other  case,  but  cylindrical  (see  Figs.  4, 
15,  16,  22,  45,  83,  96,  161,  174,  178  for  branched  leaves). 

Many  other  factors  enter  into  the  light-relation  of  foli- 
age leaves  upon  erect  stems,  but  those  given  may  suggest 


Fig.  16.    A  cycad,  showing  much-branched  leaves  and  palm-like  habit.  * 

observation  in  this  direction,  and  serve  to  show  that  the 
arrangement  of  leaves  in  reference  to  light  depends  upon 
many  things,  and  is  by  no  means  a  fixed  and  indifferent 
thing.  The  study  of  any  growing  plant  in  reference  to  this 
one  relation  presents  a  multitude  of  problems  to  those  who 
know  how  to  observe. 


B.    On  liorizontal  stems 

23.  Examples  of  horizontal  stems,  that  is,  stems  exposed 
on  one  side  to  the  direct  light,  will  be  found  in  the  case  of 
many  branches  of  trees,  stems  prostrate  on  the  ground,  and 


FOLIAGE    LEAVES:    THE    LIGHT-RELATION 


23 


stems  against  a  support,  as  the  ivies.  It  is  only  necessary 
to  notice  how  the  leaves  are  adjusted  to  light  on  an  erect 
stem,  and  then  to  bend  the 
stem  into  a  horizontal  posi- 
tion or  against  a  support,  to 
realize  how  unfavorable  the 
same  arrangement  would 
be,  and  how  many  new  ad- 
justments must  be  made. 
The  leaf  blades  must  all  be 
brought  to  the  light  side  of 
the  stem,  so  far  as  possible, 
and  those  that  belong  to 
the  lower  side  of  the  stem 
must  be  fitted  into  the 
spaces  left  by  the  leaves 
which  belong  to  the  upper 
side.  This  may  be  brought 
about  by  the  twisting  of 
the  stem,  the  twisting  of 
the  petioles,  the  bending  of 
the  blade  on  the  petiole, 
the  lengthening  of  petioles, 
or  in  some  other  way. 
Every  horizontal  stem  has 
its  own  special  problems  of 
leaf  adjustment  which  may 
be  observed  (see  Figs.  18, 
50). 

Sometimes  there  is  not 
space  enough  for  the  full 
development  of  every  blade, 
and  smaller  ones  are  fitted 
into  the  spaces  left  by  the  larger  ones  (see  Fig.  21).  This 
sometimes  results  in  what  are  called  unequally  paired  leaves, 
where  opposite  leaves  develop  one  large  blade  and  one  small 


Fig.  17.  A  chrysanthemum,  showing 
lobed  leaves,  the  rising  of  the  petioles 
to  adjust  the  blades  to  light,  and  the 
general  cylindrical  habit. 


24 


PLANT  STUDIES 


one.  Perhaps  the  most  complete  fitting  together  of  leaves 
is  found  in  certain  ivies,  where  a  regular  layer  of  angular 
interlocking  leaves  is  formed,  the  leaves  fitting  together  like 


Fig.  18.    A  plant  {Pellionia)  with  drooping  stems,  showing  how  the  leaves  are  all 
brought  to  the  lighted  side  and  fitted  together. 

the  pieces  of  a  mosaic.  In  fact  such  an  arrangement  is 
known  as  the  mosaic  arrangement,  and  involves  such  an 
amount  of  twisting,  displacement,  elongation   of  petioles. 


^'^mixi^ 


'{^1 


26 


PLANT  STUDIES 


Fig.  20.  A  spray  of  maple,  showing  the  adjustmeut  of  the  leaves  in  size  and  position 
of  blades  and  length  of  petioles  to  secure  exposure  to  light  on  a  horizontal  stem.— 
After  Kerxer. 

etc.,  as  to  give  ample  evidence  of  the  effort  put  forth  by 
plants  to  secure  a  favorable  light-relation  for  their  foliage 


Fig.  21.  Two  plants  showing  adjustment  of  leaves  on  a  horizontal  stem.  The  plant 
to  the  left  is  nightshade,  in  which  small  blades  are  fitted  into  spaces  left  by  the 
large  ones.  The  plant  to  the  right  is  Selaginella,  in  which  small  leaves  are  dis- 
tributed along  the  sides  of  the  stem,  and  others  are  displayed  along  the  upper  sur- 
face.—After  Kerner. 


FOLIAGE    leaves:    THE   LICxHT-RELaTION 


27 


leaves  (see  Figs.  19,  22).  In  the  case  of  ordinary  shade  trees 
every  direction  of  branch  may  be  found,  and  the  resulting 
adjustment  of  leaves  noted  (see  Fig.  20). 

Looking  up  into  a  tree  in  full  foliage,  it  will  be  noticed 
that  the  horizontal  branches  are   comparatively  bare  be- 


FiG.  22.    A  mosaic  of  fern  (Adianfum)  leaflets. 


neath,  wliile  the  leaf  blades  have  been  carried  to  tlie  upper 
side  and  have  assumed  a  mosaic  arrangement. 

Sprays  of  maidenhair  fern  (see  Fig.  22)  show  a  remark- 
able amount  of  adjustment  of  tlie  leaflets  to  the  light  side. 
Another  group  of  fern-plants,  known  as  club-mo'^ses,  has 
horizontal  stems  clothed  with  numerous  very  siuall  leaves. 
Tliese  leaves  may  be  seen  taking  advantage  of  all  tlie  space 
on  the  lighted  side  (see  Fig.  21). 


CHAPTER  III 

FOLIAGE    LEAVES:    FUNCTION,    STRUCTURE,    AND    PROTEC- 
TION 

A.     Functions  of  foliage  leaves 

24.  Functions  in  general. — We  have  observed  that  foliage 
leaves  are  light-related  organs,  and  that  this  relation  is  an 
important  one  is  evident  from  the  various  kinds  of  adjust- 
ment used  to  secure  it.  We  infer,  therefore,  that  for  some 
important  function  of  these  leaves  light  is  necessary.  It 
would  be  hasty  to  suppose  that  light  is  necessary  for  every 
kind  of  work  done  by  a  foliage  leaf,  for  some  forms  of  work 
might  be  carried  on  by  the  leaf  that  light  neither  helps  nor 
hinders.  Foliage  leaves  are  not  confined  to  one  function, 
but  are  concerned  in  a  variety  of  processes,  all  of  which 
have  to  do  with  the  great  work  of  nutrition.  Among  the 
variety  of  functions  which  belong  to  foliage  leaves  some  of 
the  most  important  may  be  selected  for  mention.  It  will 
be  possible  to  do  little  more  than  indicate  these  functions 
until  the  plant  with  all  its  organs  is  considered,  but  some 
evidence  can  be  obtained  that  various  processes  are  taking 
place  in  the  foliage  leaf. 

25.  Photosynthesis. — The  most  imjoortant  function  of  the 
foliage  leaf  may  be  detected  by  a  simple  experiment.  If 
an  actively  growing  water  plant  submerged  in  water  in  a 
glass  vessel  be  exposed  to  bright  light,  bubbles  may  be  seen 
coming  from  the  leaf  surfaces  and  rising  through  the  water 
(see  Fig.  23).  The  water  is  merely  a  device  by  which  the 
bubbles  of  gas  may  be  seen.     If  the  plant  is  very  active  the 

28 


FOLIAGE  LEAVES:    FUNCTION,  STRCCTURE,  ETC. 


29 


bubbles  are  numerous.  That  this  activity  holds  a  definite 
relation  to  light  may  be  proved  by  shading  the  vessel  con- 
taining the  plant.  When  the  light  is  diminished  the  bub- 
bles diminish  in  number,  and  when  sufficiently  darkened 


^jyB^tirrM 


Fig.  23.    An  expcrirat'iit  to  illustrate  the  giving  off  of  oxygen  in  the  procet^s  of  photo- 
synthesis, 

the  bubbles  will  cease  entirely.  If  now  the  vessel  be  again 
illuminated,  the  bubbles  will  reappear,  and  the  rapidity 
with  which  the  bubbles  are  formed  will  indicate  in  a  rough 
way  the  activity  of  the  process.  That  this  gas  being  given 
off  is   mainly   oxygen   may   be   proved   by  collecting   the 


30  PLANT   STUDIES 

bubbles  (by  inverting  over  the  plants  a  large  funnel  and 
leading  them  into  a  test  tube),  and  testing  it  in  the  usual 
way. 

Some  very  important  things  are  learned  by  this  experi- 
ment. It  is  evident  that  some  process  is  going  on  within 
the  leaves  that  needs  light  and  which  results  in  giving  off 
oxygen.  It  is  further  evident  that  as  oxygen  is  eliminated, 
the  process  indicated  is  dealing  with  substances  which 
contain  more  oxygen  than  is  needed.  The  amount  of 
oxygen  given  off  may  be  taken  as  the  measure  of  the  work. 
The  more  oxygen,  the  more  work ;  and,  as  we  have  observed, 
the  more  light,  the  more  oxygen;  and  no  light,  no  oxygen. 
Therefore,  light  must  be  essential  to  the  work  of  which  the 
elimination  of  oxygen  is  an  external  indication.  That  this 
process,  whatever  it  may  be,  is  so  essentially  related  to 
light,  suggests  the  idea  that  it  is  the  special  process  which 
demands  that  the  leaf  shall  be  a  light-related  organ.  If  so, 
it  is  a  dominating  kind  of  work,  as  it  chiefly  determines 
the  life-relations  of  foliage  leaves. 

The  process  thus  indicated  is  known  as  iihotosyntliesis, 
and  the  name  suggests  that  it  has  to  do  with  the  arrange- 
ment of  material  with  the  help  of  light.  It  is  really  a  pro- 
cess of  food  manufacture,  by  which  raw  materials  are  made 
into  plant  food.  This  process  is  an  exceedingly  important 
one,  for  upon  it  depend  the  lives  of  all  plants  and  animals. 
The  foliage  leaves  may  be  considered,  therefore,  as  special 
organs  of  pliotosyntliesis.  They  are  special  organs,  not  ex- 
clusive organs,  for  any  green  tissue,  whether  on  stem  or  fruit 
or  any  part  of  the  plant  body,  may  do  the  same  work.  It 
is  at  once  apparent,  also,  that  during  the  night  the  process 
of  photosynthesis  is  not  going  on,  and  therefore  during  the 
night  oxygen  is  not  being  given  off. 

Another  part  of  this  j^rocess  is  not  so  easily  observed,  but 
is  so  closely  related  to  the  elimination  of  oxygen  that  it 
must  be  mentioned.  Carbon  dioxide  occurs  in  the  air  to 
which  the  foliage  leaves  are  exposed.     It  is  given  off  from 


FOLIAGE    LEAVES:    FUNCTION,    STRUCTURE,    ETC.       31 

our  lungs  in  breathing,  and  also  comes  off  from  burning 
wood  or  coal.  It  is  a  common  waste  product,  being  a  com- 
bination of  carbon  and  oxygen  so  intimate  that  the  two 
elements  are  separated  from  one  another  Avith  great  dif- 
ficulty. During  the  process  of  photosynthesis  it  has  been 
discovered  that  carbon  dioxide  is  being  absorbed  from  the 
air  by  the  leaves.  As  this  gas  is  absorbed  chiefly  by  green 
parts  and  in  the  light,  in  just  the  conditions  in  which  oxy- 
gen is  being  given  off,  it  is  natural  to  connect  the  two,  and 
to  infer  that  the  process  of  photosynthesis  involves  not  only 
the  green  color  and  the  light,  but  also  the  absorption  of 
carbon  dioxide  and  the  elimination  of  oxygen. 

When  we  observe  that  carbon  dioxide  is  a  combination 
of  carbon  and  oxygen,  it  seems  reasonable  to  suppose  that 
the  carbon  and  oxygen  are  separated  from  one  another  in 
the  plant,  and  that  the  carbon  is  retained  and  the  oxygen 
given  back  to  the  air.  The  process  of  photosynthesis  may 
be  partially  defined,  therefore,  as  the  breaking  up  of  carbon 
dioxide  by  the  green  parts  of  the  plants  in  the  presence  of 
light,  the  retention  of  the  carbon,  and  the  elimination  of 
the  oxygen.  The  carbon  retained  is  combined  into  real 
plant  food,  in  a  way  to  be  described  later.  AVe  may  con- 
sider photosynthesis  as  the  most  important  function  of  the 
foliage  leaf,  of  which  the  absorption  of  carbon  dioxide  and 
the  evolution  of  oxygen  are  external  indications  ;  and  that 
light  and  chlorophyll  are  in  some  way  essentially  connected 
with  it. 

20.  Transpiration. — One  of  tlie  easiest  things  to  observe 
in  connection  with  a  working  leaf  is  the  fact  that  it  gives 
off  moisture.  A  simple  experiment  may  demonstrate  this. 
If  a  glass  vessel  (bell  jar)  be  inverted  over  a  small  active 
plant  the  moisture  is  seen  to  condense  on  the  glass,  and 
even  to  trickle  down  the  sides.  A  still  more  convenient  way 
to  demonstrate  this  is  to  select  a  single  vigorous  leaf  with 
a  good  petiole  ;  pass  the  petiole  through  a  perforated  card- 
board resting  upon  a  tumbler  containing  water,  and  invert 


32  PLANT  STUDIES 

a  second  tumbler  over  the  blade  of  the  leaf,  which  projects 
above  the  cardboard  (see  Fig.  24).  It  will  be  observed  that 
moisture  given  off  from  the  surface  of  the  working  leaf  is 
condensed  on  the  inner  surface  of  the  inverted  tumbler. 
The  cardboard  is  to  shut  off  evaporation  from  the  water 
in  the  lower  tumbler. 

When  the  amount  of  water  given  off  by  a  single  leaf  is 
noted,  some  vague  idea  may  be  formed  as  to  the  amount  of 
moisture  given  off  by  a  great  mass  of  vegetation,  such  as  a 
meadow  or  a  forest.  It  is  evident  that  green  plants  at 
work  are  contributing  a  very  large  amount  of  moisture  to 
the  air  in  the  form  of  water  vapor,  moisture  which  has 
been  absorbed  by  some  region  of  the  plant.  The  foli- 
age leaf,  therefore,  may  be  regarded  as  an  organ  of 
transpiration^  not  that  the  leaves  alone  are  engaged  in 
transpiration,  for  many  parts  of  the  plant  do  the  same 
thing,  but  because  the  foliage  leaves  are  the  chief  seat  of 
transpiration. 

In  case  the  leaves  are  submerged,  as  is  true  of  many 
plants,  it  is  evident  that  transpiration  is  practically  checked, 
for  the  leaves  are  already  bathed  with  water,  and  under  such 
circumstances  water  vapor  is  not  given  off.  It  is  evident 
that  under  such  circumstances  leaf  work  must  be  carried 
on  without  transpiration.  In  some  cases,  as  in  certain 
grasses,  fuchsias,  etc.,  drops  of  water  are  extruded  at  the 
apex  of  the  leaf,  or  at  the  tips  of  the  teeth.  This  process 
is  called  guttation^  and  by  means  of  it  a  good  deal  of 
water  passes  from  the  leaf.  It  is  specially  used  by  shade 
plants,  which  live  in  conditions  that  do  not  favor  tran- 
spiration. 

27.  Respiration. — Another  kind  of  work  also  may  be 
detected  in  the  foliage  leaf,  but  not  so  easily  described. 
In  fact  it  escaped  the  general  attention  of  botanists  much 
longer  than  did  photosynthesis  and  transpiration.  It  is 
work  that  goes  on  so  long  as  the  leaf  is  alive,  never  ceasing 
day  or  night.    The  external  indication  of  it  is  the  absorption 


Fig.  24.     Experiment  illustrating  transpiration. 


34  PLANT   STUDIES 

of  oxygen  and  the  giving  out  of  carbon  dioxide.  It  will  be 
noted  at  once  that  this  is  exactly  the  reverse  of  what  takes 
place  in  photosynthesis.  During  the  day,  therefore,  carbon 
dioxide  and  oxygen  are  both  being  absorbed  and  evolved. 
It  will  also  be  noted  that  the  taking  in  of  oxygen  and  the 
giving  out  of  carbon  dioxide  is  just  the  sort  of  exchange 
which  takes  place  in  our  own  resjiiration.  In  fact  this  pro- 
cess is  also  called  respiration  in  plants.  It  does  not  depend 
upon  light,  for  it  goes  on  in  the  dark.  It  does  not  depend 
upon  chlorophyll,  for  it  goes  on  in  plants  and  parts  of  plants 
which  are  not  green.  It  is  not  peculiar  to  leaves,  but  goes 
on  in  every  living  part  of  the  plant.  A  process  which  goes 
on  without  interruption  in  all  living  plants  and  animals 
must  be  very  closely  related  to  their  living.  We  conclude, 
therefore,  that  while  photosynthesis  is  peculiar  to  green 
plants,  and  only  takes  j)lace  in  them  when  light  is  present, 
respiration  is  necessary  to  all  plants  in  all  conditions,  and 
that  when  it  ceases  life  must  soon  cease.  The  fact  is, 
respiration  supplies  the  energy  which  enables  the  living 
substance  to  work. 

Once  it  was  thought  that  plants  differ  from  animals 
in  the  fact  that  plants  absorb  carbon  dioxide  and  give  off 
oxygen,  while  animals  absorb  oxygen  and  give  off  carbon 
dioxide.  It  is  seen  now  that  there  is  no  such  difference, 
but  that  respiration  (absorption  of  oxygen  and  evolution  of 
carbon  dioxide)  is  common  to  both  plants  and  animals. 
The  difference  is  that  green  plants  have  the  added  work  of 
photosynthesis. 

We  must  also  think  of  the  foliage  leaf,  therefore,  as  a 
respiring  organ,  because  very  much  of  such  work  is  done 
by  it,  but  it  must  be  remembered  that  respiration  is  going 
on  in  every  living  part  of  the  plant. 

This  by  no  means  completes  the  list  of  functions  that 
might  be  made  out  for  foliage  leaves,  but  it  serves  to  indi- 
cate both  their  peculiar  work  (photosynthesis)  and  the  fact 
that  they  are  doing  other  kinds  of  work  as  well. 


FOLIAGE   LEAVES  :    FUNCTION,    STRUCTURE,    ETC.       35 

B.   Structure  of  foliage  leaves 

28.  Gross  structure. — It  is  evident  that  the  essential  part 
of  a  foliage  leaf  is  its  expanded  portion  or  Uade.    Often  the 

leaf  is  all  blade  (see  Figs.  7, 
8, 18)  ;  frequently  there  is  a 
longer  or  shorter  leaf-stalk 
{petiole)  which  helps  to  put 


Pig.  25.  Two  t\  jks  of  leaf  venation.  The  figure  to  the  left  is  a  leaf  of  Solomon's 
seal  {Pnlygouatum),  and  shows  the  principal  veins  parallel,  the  very  minute  cross 
veinlets  being  invisible  to  the  naked  eye,  being  a  monocotyl  type.  The  figure  to 
.the  right  is  a  leaf  of  a  willow,  and  shows  netted  veins,  the  main  central  vein  (mid- 
rib) sending  out  a  series  of  parallel  branches,  which  are  connected  with  one  another 
by  a  network  of  veinlets,  being  a  dicotyl  type.— After  Ettingsuausen. 

the  blade  into  better  light-relation  (see  Figs.  1,  9,  17,  20, 
2G);  and  sometimes  there  are  little  leaf -like  aj^pendages  {stip- 
ules) on  the  petiole  where  it  joins  the  stem,  whose  func- 
tion is  not  always  clear.  Upon  examining  tlie  blade  it 
is  seen  to  consist  of  a  green  substance   through  which  a 


36 


PLANT   STUDIES 


framework  of  veins  is  variously  arranged.  The  large  veins 
which  enter  the  blade  send  off  smaller  branches,  and  these 
send  off  still  smaller  ones,  until  the  smallest  veinlets  are 

invisible,  and  the 
framework  is  a 
close  network  of 
branching  veins. 
This  is  plainly 
shown  by  a  ''skel- 
eton "  leaf,  one 
which  has  been  so 
treated  that  all 
the  green  sub- 
stance has  disap- 
peared, and  only 
the  network  of 
veins  remains.  It 
will  be  noticed 
that  in  some 
leaves  the  veins 
and  veinlets  are 
very  prominent, 
in  others  only 
the  main  veins 
are  prominent, 
while  in  some  it 
is  hard  to  detect 
any  veins  (see 
Figs.  25,  26). 

29.  Significance 
of  Isaf  veins. — It 
is  clear  that  the 
framework  of  veins  is  doing  at  least  two  things  for  the 
blade:  (1)  it  mechanically  supports  the  sjoread  out  green  sub- 
stance ;  and  (2)  it  conducts  material  to  and  from  the  green 
substance.      So  complete  is  the  network  of  veins  that  this 


Fig.  26.  A  leaf  of  hawthorn,  showing  a  short  petiole,  and 
a  broad  toothed  blade  with  a  conspicuous  network  of 
veins.  Note  the  relation  between  the  veins  and  the 
teeth.— After  Stkasburger. 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.       37 

support  and  conduction  are  very  perfect  (see  Fig.  27).  It 
is  also  clear  that  the  green  substance  thus  supported  and 
supplied  with  material  is  the  important  part  of  the  leaf,  the 
part  that  demands  the  light-relation.  Study  the  various 
plans  of  the  vein  systems  in  Figs.  3,  9,  13,  18,  19,  20,  21, 
25,  2Q,  51,  70,  73,  82,  83,  92,  161. 


Fio.  2f.   A  plant  (Fittonia)  whose  leaves  show  a  network  of  veins,  and  also  an  adjas^ 
ment  to  one  another  to  form  a  mosaic. 

30.  Epidermis. — If  a  thick  leaf  be  taken,  such  as  that 
of  a  hyacinth,  it  will  be  found  possible  to  peel  off  from 
its  surface  a  delicate  transparent  skin  {epidermis).  This 
epidermis  completely  covers  the  leaf,  and  generally  shows 
no  green  color.  It  is  a  protective  covering,  but  at  the  same 
time  it  must  not  completely  shut  off  the  green  substance 
beneath  from  the  outside.  It  is  found,  therefore,  that 
three  important  parts  of  an  ordinary  foliage  leaf  are  :  (1) 


38 


PLANT   STUDIES 


Fig.  28.  Cells  of  the  epidermis 
of  Maranta,  showing  the 
interlocking  walls,  and  a 
stoma  {s)  with  its  two  guard- 
cells. 


a  network  of  veins  ;  (2)  a  green  substance   {mesopliyll)  in 

the  meshes  of  the  network  ;  and  (3)  over  all  an  epidermis. 
31.  Stomata. — If  a  compound  microscope  is  used,  some 

very  important  additional  facts  may  be  discovered.  The 
thin,  transparent  epidermis  is 
found  to  be  made  up  of  a  layer  of 
cells  which  fit  closely  together, 
sometimes  dovetailing  with  each 
other.  Curious  openings  in  the 
epidermis  will  also  be  discovered, 
sometimes  in  very  great  numbers. 
Guarding  each  opening  are  two 
crescent-shai)ed  cells,  known  as 
^?^ar^-cells,  and  between  them  a 
slit-like  opening  leads  through  the 
e^^idermis.  The  whole  apparatus 
is  known  as  a  sto7iia  (plural 
stomata),     which     really     means 

''mouth,"  of  which  the  guard-cells  might  be  called  the 

lips  (see  Figs.  28,  29).     Sometimes  stomata  are  found  only 

on  the  under  side  of  the  leaf,  sometimes  only 

on  the  upper  side,  and  sometimes  on  both 

sides. 

One  important  fact  about  stomata  is  that 

the  guard-cells  can  change  their  shape,  and 

so  regulate  the  size  of  the  opening.    It  is  not 

certain  just  why  the  guard-cells  change  their 

shape  and  just  what  stomata  do  for  leaves. 

They  are  often  called  "  breathing  pores,"  but 

a  better  name  would  be  air  pores.     Stomata 

are  not  peculiar  to  the  epidermis  of  foliage 

leaves,  for  they  are  found  in  the  epidermis 

of  any  green  part,  as  stems,  young  fruit, 

etc.     It  is  evident,  therefore,  that  they  hold 

an  important  i  elation  to  green  tissue  which 

is  covered  by  epidermis.    Also,  if  we  examine 


Fig.  29.  A  single 
stoma  from  the 
epidermis  of  a 
lily  leaf,  show- 
ing the  two 
guard-cells  full 
of  chlorophyll, 
and  the  smell 
slit-like  opening 
between. 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.        39 

foliage  leaves  and  other  green  parts  of  plants  which  live 
submerged  in  water,  we  find  that  the  epidermis  contains 
no  stomata.  Therefore,  stomata  hold  a  definite  relation 
to  green  parts  covered  by  epidermis  only  when  this  epider- 
mis is  exposed  to  the  air. 

It  would  seem  that  the  stomata  supply  open  passage- 
ways for  material  from  the  green  tissue  through  the  epider- 
mis to  the  air,  or  from  the  air  to  the  green  tissue,  or  both. 
It  will  be  remembered,  however,  that  quite  a  number  of 
substances  are  taken  into  the  leaf  and  given  out  from  it, 
so  that  it  is  hard  to  determine  whether  the  stomata  are 
specially  for  any  one  of  these  movements.  For  instance, 
the  leaf  gives  out  moisture  in  transpiration,  oxygen  in 
photosynthesis,  and  carbon  dioxide  in  respiration  ;  while  it 
takes  in  carbon  dioxide  in  photosynthesis,  and  oxygen  in 
respiration.  It  is  thought  that  stomata  specially  favor 
transpiration,  and  that  they  also  much  facilitate  the  en- 
trance of  carbon  dioxide. 

32.  Mesophyll. — If  a  cross-section  be  made  of  an  ordi- 
nary foliage  leaf,  such  as  that  of  a  lily,  the  three  leaf 
regions  can  be  seen  in  their  proper  relation  to  each  other. 
Bounding  the  section  above  and  below  is  the  layer  of  trans- 
parent epidermal  cells,  pierced  here  and  there  by  stomata, 
marked  by  their  peculiar  guard-cells.  Between  the  epi- 
dermal layers  is  the  green  tissue,  known  as  the  mesophyll, 
made  up  of  cells  which  contain  numerous  small  green 
bodies  which  give  color  to  the  whole  leaf,  and  are  known  as 
chlorophyll  bodies  or  chloroplasts. 

The  mesophyll  cells  are  usually  arranged  differently  in 
the  upper  and  lower  regions  of  the  leaf.  In  the  upper 
region  the  cells  are  elongated  and  stand  upright,  present- 
ing their  narrow  ends  to  the  upper  leaf  surface,  forming 
the  palisade  tissue.  In  the  lower  region  the  cells  are  irreg- 
ular, and  so  loosely  arranged  as  to  leave  passageways  for  air 
between,  forming  tlie  spo7igy  tissue.  The  air  spaces  among 
the  cells  communicate  with  one  another,  so  that  a  system  of 
4 


40 


PLANT   STUDIES 


air  chambers  extends  throughout  the  spongy  mesophyll. 
It  is  into  this  system  of  air  chambers  that  the  stomata 
open,  and  so  they  are  put  into  direct  communication  with 
the  mesophyll  or  working  cells.  The  peculiar  arrangement 
of  the  upper  mesophyll,  to  form  the  palisade  tissue,  has  to 
do  with  the  fact  that  that  surface  of  the  leaf  is  exposed  to 
the  direct  rays  of  light.  This  light,  so  necessary  to  the 
mesophyll,  is  also  dangerous  for  at  least  two  reasons.     If 


FxG.  30.  A  section  through  the  leaf  of  lily,  showing  upper  epidermis  (.tie),  lower  epi- 
dermis (le)  with  its  stomata  (st),  mesophyll  (dotted  cells)  composed  of  the  palisade 
region  (p)  and  the  spongy  region  (sp)  with  airspaces  among  the  cells,  and  two 
veins  (r)  cut  across. 


the  light  is  too  intense  it  may  destroy  the  chlorophyll,  and 
the  heated  air  may  dry  out  the  cells.  The  narrow  ends  of 
the  cells  present  less  exposure,  and  the  depth  of  the  cells 
perm.its  greater  freedom  of  movement  to  the  chloroplasts. 

3B.  Veins. — In  the  cross-section  of  the  leaf  there  will 
also  be  seen  here  and  there,  embedded  in  the  mesophyll, 
the  cut  ends  of  the  veinlets,  made  up  partly  of  thick- 
walled  cells,  which  hold  the  leaf  in  shape  and  conduct 
material  to  and  from  the  mesophyll  (see  Fig.  30). 


FOLIAGE  LEAVES:    FUNCTION,   STRUCTURE,    ETC.       41 


0.    Leaf  protection 

34.  Need  of  protection. — Such  an  important  organ  as 
the  leaf,  with  its  delicate  active  cells  well  displayed,  is  ex- 
posed to  numerous  dangers.  Chief  among  these  dangers 
are  intense  light,  drought,  and  cold.  All  leaves  are  not 
exposed  to  these  dangers.  For  example,  plants  which  grow 
in  the  shade  are  not  in  danger  from  intense  light  ;  many 
^^  water  plants  are  not  in  danger 

from  drought ;  and  ^^lants  of 
the  tropical  lowlands  are  in  no 


Fig.  31.  Sections  tlirough  leaves  of  the  same  plant,  showing  the  effect  of  exposure  to 
light  upon  the  structure  of  the  mesophyll.  In  both  cases  os  indicates  upper  surface, 
and  us  under  surface.  In  the  section  at  the  left  the  growing  leaf  was  exposed  to 
direct  and  intense  sunlight,  and,  as  a  consequence,  all  of  the  mesophyll  cells  have 
assumed  the  protected  or  palisade  position.  In  the  section  at  the  right  the  leaf  was 
grown  in  the  shade,  and  none  of  the  mesophyll  cells  have  organized  in  palisade 
fashion. — After  Stahl. 

danger  from  cold.  The  danger  from  all  these  sources  is  be- 
cause of  the  large  surface  with  no  great  thickness  of  body, 
and  the  protection  against  all  of  them  is  practically  the 
same.  Most  of  the  forms  of  protection  can  bo  reduced 
to  two  general  plans:  (1)  the  development  of  protective 
structures  between  the  endangered  mesophyll  and  tlie  air  ; 
(2)  the  diminution  of  the  exposed  surface. 

35.  Protective  structures. — The  palisade  arrangement  of 
mesophyll  may  be  regarded  as  an  adaptation  for  protection, 


42 


PLANT   STUDIES 


but  it  usually  occurs,  and  does  not  necessarily  imply  ex- 
treme conditions  of  any  kind.  However,  palisade  tissue  of 
unusually  narrow  and  elongated  cells,  or  forming  two  or 


Fig.  32.    Section  through  a  portion  of  the  leaf  of  the  yew  (Taxus),  showing  cuticle 
(c),  epidermis  (e),  and  the  upper  portion  of  the  palisade  cells  {p). 

three  layers,  indicates  exposure  to  intense  light  or  drouth, 
and  is  very  characteristic  of  alpine  and  desert  plants.  The 
accompanying  illustration  (Fig.  31)  shows  in  a  striking 
way  the  effect  of  light  intensity  upon  the  structure  of  the 
mesophyll,  by  contrasting  leaves  of  the  same  plant  exposed 
to  the  extreme  conditions  of  light  and  shade. 

The  most  usual  structural  adaptations,  however,  are 
connected  with  the  epidermis.  The  outer  walls  of  the  epi- 
dermal cells  may  become  thickened,  sometimes  excessively 

so ;  the  other  epidermal 
walls  may  also  become 
more  or  less  thickened ; 
or  even  what  seems  to 
be  more  than  one  epi- 
dermal layer  is  found 
protecting  the  meso- 
phyll.    If    the    outer 

Fig.  33.    Section  through  a  portion  of  the  leaf  of   walls    of    the     epidermal 
carnation,   showing   the   heavy   cuticle    (cu)      ^j  continue      tO 

formed  by  the  outer  walls  of  the  epidermal 

cells  (qo).  Through  the  cuticle  a  passageway  thicken,  the  OUtcr  rC- 
leads  to  the  stoma,  whose  two  guard-cells  are  q-Jq-q  ^f  ^\^q  thick  Wall 
seen  lying  between  the  two  epidermal  cells   ^ 

shown  in  the  figure.  Below  the  epidermal  loSCS  its  strUCturC 
cells  some  of  the  palisade  cells  (paO  are  shown  ^^^^  formS  the  CuHcle, 
containing  chloroplasts,  and  below  the  stoma        i    •     i       •  -P     +V. 

is  seen  the  air  chamber  into  which  it  opens.       WlllCh    IS     OUG     01     the 


FOLIAGE   LEAVES:    FUNCTION,    STRUCTURE,    ETC.       43 


Fig.  34.  A  hair  from  the  leaf 
of  Potentilla.  It  is  seen 
to  grow  out  from  the  epi- 
dermis. 


best  protective  substances  (see  Fig. 
32).  Sometimes  this  cuticle  be- 
comes so  thick  tliat  the  passage- 
ways through  it  leading  down  to 
the  stomata  become  regular  canals 
(see  Fig,  33). 

Another  very  common  protective 
structure  upon  leaves  is  to  be  found 
in  the  great  variety  of  hairs  de- 
veloped by  the  epidermis.  These 
may  form  but  a  slightly  downy 
covering,  or  the  leaf  may  be  cov- 
ered by  a  woolly  or  felt-like  mass 
so  that  the  epidermis  is  entirely 
concealed.  The  common  mullein 
is  a  good  illustration  of  a  felt- 
covered  leaf  (see  Fig.  3G).  In  cold 
or  dry  regions  the  hairy  covering 
of  leaves  is  very  noticeable,  often 
giving  them  a  brilliant  silky  white  or  bronze  look  (see 
Figs.  34,  35).  Sometimes,  instead  of  a  hair-like  cover- 
ing, the  epidermis  develops  scales  of  various  patterns, 
often  overlapping,  and  forming  an  excellent  protection 
(see  Fig.  37).  In  all  these  cases  it  should  be  remembered 
that  these  hairs  and  scales  may  serve  other  purposes  also, 
and  may  even  be  of  no  use  whatever  to  the  plant. . 

30.   Diminution 
of  exposed  surface. —     OOp^ 
It  will  be  impossible     Ji^^i^-^^^^'^irU^r^^^^^ 
to  give  more  than  a 
few  illustrations  of 
this    large   subject. 

In   verv  drv  reo-ions      ^'°'  ^'    ^  ^^*'°°  through  the  leaf  of  bush  clover 

J         J        to  (Lesjyedeza),  showing  upper  and  lower  epidermis, 

it    has     always    been  palisade  cells,   and  cells  of  the  spongy  region. 

noticed      that     the  "^'^^    lower  epidermis  produces  numerous  hairs 

which  bend  sharply  and  lie  along  the  leaf  surface 

leaves  are  small  and  (appreesed),  forming  a  close  covering 


u 


PLANT   STUDIES 


Fig.  36.    A  branching  Irair  from  the  leaf  of  common  mullein,  showing  the  outline  but 
not  the  many  cells. 

comparatively  thick,  altliougli  they  may  be  very  numerous 
(see  Figs.  4,  172).     In  this  way  each  leaf  exposes  a  small 

surface  to  the  dry- 
ing air  and  intense 
sunlight.  In  our 
southwestern  dry 
regions  the  cactus 
abounds,  plants 
which  have  reduced 
their  leaves  so  much 
that  they  are  no 
longer  used  for 
chlorophyll  work, 
and  are  not  usually 
recognized  as  leaves. 
In  their  stead  the 
globular  or  cylin- 
drical or  flattened 
stems  are  erreen  and 

Fig.  37.    A  scale  from  the  leaf  of  ^SAep^crc^ia.  These  ^ 

scales  overlap  and  form  a  complete  covering.  0-0    leai   WOrii    {^r  IgS. 


a 


Fig.  39.  A  group  of  cactus  forms  (slender  cylindrical,  columnar, 
and  globular),  all  of  them  spiny  and  without  leaves  ;  an  agave  in 
front ;  clusters  of  yucca  flowers  in  the  background. 


TOLIAGE   leaves:    FUNCTION,   STRUCTURE,    ETC.       47 

38,  39,  40,  190,  191,  192,  193).  In  the  same  regions  the 
agaves  and  yuccas  retain  their  leaves,  but  they  become  so 
thick  that  they  serve  as  water  reservoirs  (see  Figs.  38,  39, 


Fig.  40.    A  globular  cactus,  showing  the  rihbed  stem,  the  strong  spines,  and  the  entire 
absence  of  leaves. 

194).  In  all  these  cases  this  reduced  surface  is  supple- 
mented by  palisade  tissue,  very  thick  epidermal  walls,  and 
an  abundant  cuticle. 

37.  Rosette  arrangement. — The  rosette  arrangement  of 
leaves  is  a  very  common  method  of  protection  used  by 


48 


PLAiNT   STUDIES 


small  plants  growing  in  exposed  situations,  as  bare  rocks 
and  sandy  ground.  The  cluster  of  leaves,  flat  upon  the 
ground,  or  nearly  so,  and  more  or  less  overlapping,  is  very 
effectively  arranged  for  resisting  intense  light  or  drought 
or  cold  (see  Figs.  11,  12,  48). 

38.  Protective  positions. — In   other  cases,  a  position  is 
assumed  by  the  leaves  whicli  directs  their  flat 
surfaces  so   that   they  are   not  exposed  to  the 
most  intense  rays  of  light.     The  so-called 


Fig.  41.  A  leaf  of  a  sensitive  plant  in  two  conditions.  In  the  figure  to  the  left  the 
leaf  is  fully  expanded,  with  its  four  main  divisions  and  numerous  leaflets  well 
spread.  In  the  figure  to  the  right  is  shown  the  same  leaf  after  it  has  been 
"shocked"  by  a  sudden  touch,  or  by  sudden  heat,  or  in  some  other  way.  The 
leaflets  have  been  thrown  together  forward  and  upward  ;  the  four  main  divisions 
have  been  moved  together ;  and  the  main  leaf-stalk  has  been  directed  sharply 
downw-ard.     The  whole  change  has  very  much  reduced  the  surface  of  exposure.— 

After  DUCHAKTKE. 


pass  plants, ^"^  already  mentioned,  are  illustrations  of  this, 
the  leaves  standing  edgewise  and  receiving  on  their  surface 
the  less  intense  rays  of  light  (see  Figs.  5,  170).  In  the 
dry  regions  of  Australia  the  leaves  on  many  of  the  forest 
trees  and  shrubs  have  this  characteristic  edgewise  position, 
known  as  the  profile  position,  giving  to  the  foliage  a  very 
curious  appearance. 

Some  leaves  have  the  power  of  shifting  their  position 
according  to  their  needs,  directing  their  flat  surfaces  to- 
ward the  light,  or  more  or  less  inclining  them,  according 


Fig.  42.  The  tolearaph  plant  {Desmodinm  gyrans).  Each  leaf  is  made  up  of  three 
leallets,  a  large  terminal  one,  and  a  pair  of  small  lateral  ones.  lu  the  lowest  figure 
the  large  leaflets  are  spread  out  in  their  day  position  ;  in  the  central  figure  they  are 
turned  sharply  downward  in  their  night  position.  The  name  of  the  plant  refers  to 
the  peculiar  and  constant  motion  of  the  pair  of  lateral  leaflets,  each  one  of  which 
describes  a  curve  with  a  jerking  motion,  like  the  second-hand  of  a  watch,  as 
Indicated  in  the  uppermost  figure. 


50 


PLANT   STUDIES 


to  the  danger.  Perhaps  the  most  completely  adapted 
leaves  of  this  kind  are  those  of  the  '^sensitive  plants/' 
whose  leaves  respond  to  various  external  influences  by 
changing  their  positions.  The  common  sensitive  plant 
abounds  in  dry  regions,  and  may  be  taken  as  a  type  of 
such  plants  (see  Figs.  4,  41,  171).  The  leaves  are  divided 
into  very  numerous  small  leaflets,  sometimes  very  small, 
which  stretch  in  pairs  along  the  leaf  branches.  When 
drought  approaches,  some  of  the  pairs  of  leaflets  fold  to- 
gether, slightly  reduc- 
ing the  surface  expo- 
sure. As  the  drought 
continues,  more  leaflets 
fold  together,  then  still 
others,  until  finally  all 
the  leaflets  may  be 
folded  together,  and  the 
leaves  themselves  may 
bend  against  the  stem. 
It  is  like  a  sailing  vessel 
gradually  taking  in  sail 
as  a  storm  approaches,  until  finally  nothing  is  exposed, 
and  the  vessel  weathers  the  storm  by  presenting  only  bare 
poles.  Sensitive  plants  can  thus  regulate  the  exjoosed  sur- 
face very  exactly  to  the  need. 

Such  motile  leaves  not  only  behave  in  this  manner  at  the 
coming  of  drought,  but  the  positions  of  the  leaflets  are 
shifted  throughout  the  day  in  reference  to  light,  and  at 
night  a  very  characteristic  position  is  assumed  (see  Figs.  2, 
3, 42),  once  called  a  "  sleeping  position."  One  danger  from 
night  exposure  may  come  from  the  radiation  of  heat  which 
might  chill  the  leaves  too  much ;  but  the  night  position 
may  have  no  such  meaning.  The  leaflets  of  Oxalis  have 
been  referred  to  (see  §14).  Similar  changes  in  the  direc- 
tion of  the  leaf  planes  at  the  coming  of  night  may  be 
observed  in  most  of  the  Leguminosce,  even  the  common 


Fig.  43.  Cotyledons  of  squash  seedling,  show- 
ing positions  iu  light  (left  figure)  and  in 
darkness  (right  figure).— After  Atkinson. 


FOLIAGE   LEAVES  :    FUNCTION,   STRUCTURE,    ETC.       51 


white  clover  displaying  it.  It  can  be  observed  that  the 
expanded  seed  leaves  (cotyledons)  of  many  young  germinat- 
ing plants  shift  their  positions  at  night  (see  Fig.  43),  often 
assuming  a  vertical  position  which  brings  them  in  contact 
with  one  another,  and  also  covers  the  stem  bud  (ijlumule). 

Certain  leaves  with  well-developed 
protective  structures  are  able  to  en- 
dure the  winter,  as  in  the  case  of 
the  so-called  evergreens.  In  the 
case. of  juniper,  however,  the  winter 
and  summer  positions  of  the  leaves 
are  quite  different  (see  Fig.  44).  In 
the  winter  the  leaves  lie  close  against 
the  stem  and  overlap  one  another; 
while  with  the  coming  of  summer 
conditions  they  become  widely 
spreading. 

39.  Protection  against  rain. — It  is 
also  necessary  for  leaves  to  avoid 
becoming  wet  by  rain.  If  the  water 
is  allowed  to  soak  in  there  is  danger 
of  filling  the  stomata  and  interfering 
with  the  air  exchanges.  Hence  it 
will  be  noticed  that  most  leaves  are 
able  to  shed  water,  partly  by  their 
positions,  partly  by  their  structure. 
In  many  plants  the  leaves  are  so  ar- 
ranged that  the  water  runs  off  towards  the  stem  and  so 
reaches  the  main  root  system  ;  in  other  plant's  the  rain  is 
shed  outwards,  as  from  the  eaves  of  a  house. 

Some  of  the  structures  which  prevent  the  rain  from 
soaking  in  are  a  smooth  epidermis,  a  cuticle  layer,  waxy 
secretions,  felt-like  coverings,  etc.  Interesting  experi- 
ments may  be  performed  with  different  leaves  to  test  their 
power  of  shedding  water.  If  a  gentle  spray  of  water  is 
allowed  to  play  upon  different  plants,  it  will  be  observed 


Fig.  44.  Two  twigs  of  juni- 
per, showing  the  ordinary- 
summer  and  winter  po- 
sitions assumed  by  the 
leaves.  The  ordinary  pro- 
tected winter  position  of 
the  leaves  is  shown  by  A; 
while  in  B,  in  response  to 
summer  conditions,  the 
leaves  have  spread  apart 
and  have  become  freely  ex- 
posed.—After  Warming. 


52  PLANT   STUDIES 

that  the  water  glances  off  at  once  from  the  surfaces  of 
some  leaves,  runs  off  more  slowly  from  others,  and  may  be 
more  or  less  retained  by  others. 

In  this  same  connection  it  should  be  noticed  that  in 
most  horizontal  leaves  the  two  surfaces  differ  more  or  less 
in  appearance,  the  upper  usually  being  smoother  than  the 
lower,  and  the  stomata  occurring  in  larger  numbers,  some- 
times exclusively,  upon  the  under  surface.  While  these 
differences  doubtless  have  a  more  important  meaning  than 
protection  against  wetting,  they  are  also  suggestive  in  this 
connection. 


CHAPTER   IV 

SHOOTS 

40.  General  characters. — The  term  shoot  is  used  to  include 
both  stem  and  leaves.  Among  the  lower  plants,  such  as 
the  alga?  and  toadstools,  there  is  no  distinct  stem  and  leaf. 
In  such  plants  the  working  body  is  spoken  of  as  the  thallus, 
which  does  the  work  done  by  both  stem  and  leaf  in  the 
higher  plants.  These  two  kinds  of  work  are  separated  in 
the  higher  plants,  and  the  shoot  is  differentiated  into  stem 
and  leaves. 

41.  Life-relation. — In  seeking  to  discover  the  essential 
life-relation  of  the  stem,  it  is  evident  that  it  is  not  neces- 
sarily a  light-relation,  as  in  the  case  of  the  foliage  leaf, 
for  many  stems  are  subterranean.  Also,  in  general,  the 
stem  is  not  an  expanded  organ,  as  is  the  ordinary  foli- 
age leaf.  This  indicates  that  whatever  may  be  its  essential 
life-relation  it  has  little  to  do  with  exposure  of  surface. 
It  becomes  plain  that  the  stem  is  the  great  leaf-bearing 
organ,  and  that  its  life-relation  is  a  leaf-relation.  Often 
stems  branch,  and  this  increases  their  power  of  producing 
leaves. 

In  classifying  stems,  therefore,  it  seems  natural  to  use 
the  kind  of  leaves  they  bear.  From  this  standpoint  there 
are  three  prominent  kinds  of  stems  :  (1)  those  bearing  foli- 
age leaves  ;  (2)  those  bearing  scale  leaves ;  and  (3)  those 
bearing  floral  leaves.  There  are  some  peculiar  forms  of 
stems  which  do  not  bear  leaves  of  any  kind,  but  they  need 
not  be  included  in  this  general  view. 

53 


64  PLANT  STUDIES 

A.     Stems  bearing  foliage  leaves. 

42.  General  character. — As  the  purpose  of  this  stem  is  to 
display  foliage  leaves^  and  as  it  has  been  discovered  that  the 
essential  life-relation  of  foliage  leaves  is  the  light-relation, 
it  follows  that  a  stem  of  this  type  must  be  able  to  relate  its 
leaves  to  light.  It  is,  therefore,  commonly  aerial,  and  that 
it  may  properly  display  the  leaves  it  is  generally  elongated, 
with  its  joints  (nodes)  bearing  the  leaves  well  separated  (see 
Figs.  1,  4,  18,  20). 

The  foliage-bearing  stem  is  generally  the  most  conspicu- 
ous part  of  the  plant  and  gives  style  to  the  whole  body. 
One's  impression  of  the  forms  of  most  plants  is  obtained 
from  the  foliage-bearing  stems.  Such  stems  have  great 
range  in  size  and  length  of  life,  from  minute  size  and  very 
short  life  to  huge  trees  which  may  endure  for  centuries. 
Branching  is  also  quite  a  feature  of  foliage-bearing  stems  ; 
and  when  it  occurs  it  is  evident  that  the  power  of  display- 
ing foliage  is  correspondingly  increased.  Certain  promi- 
nent types  of  foliage-bearing  stems  may  be  considered. 

43.  The  subterranean  type. — It  may  seem  strange  to  in- 
clude any  subterranean  stem  with  those  that  bear  foliage, 
as  such  a  stem  seems  to  be  away  from  any  light-relation. 
Ordinarily  subterranean  stems  send  foliage-bearing  branches 
above  the  surface,  and  such  stems  are  not  to  be  classed  as 
foliage-bearing  stems.  But  often  the  only  stem  possessed 
by  the  plant  is  subterranean,  and  no  branches  are  sent  to 
the  surface.  In  such  cases  only  foliage  leaves  appear  above 
ground,  and  they  come  directly  from  the  subterranean  stem. 
The  ordinary  ferns  furnish  a  conspicuous  illustration  of 
this  habit,  all  that  is  seen  of  them  above  ground  being  the 
characteristic  leaves,  the  commonly  called  "  stem ''  being 
only  the  petiole  of  the  leaf  (see  Figs.  45,  46,  144).  Many 
seed  plants  can  also  be  found  which  show  the  same  habit, 
especially  those  which  flower  early  in  the  spring.  This 
cannot  be   regarded  as  a  very  favorable  type  of  stem  for 


!>^(^^>-7'    '    .5^^  '""'  "^ 


Fig.  45.  A  fern  Uspidium),  showing  three  large  branching  leaves  coming  from  a  hori- 
zontal snbtcrranean  stem  (rootstock) ;  growing  leaves  are  also  shown,  which  are 
gradually  unrolling.  The  stem,  young  leaves,  and  petioles  of  the  large  leaves  are 
thickly  covered  with  protecting  hairs.  The  stem  gives  rise  to  numerous  small  roots 
from  Its  lower  surface.  The  figure  marked  3  represents  the  under  surface  of  a 
portion  of  the  leaf,  showing  seven  groups  of  siiore  cases  ;  at  5  is  represented  a 
section  through  one  of  these  groups,  showing  how  the  spore  cases  are  attached  and 
protected  l)y  a  flap  ;  while  at  6  is  represented  a  single  spore  case  opening  and  dis- 
charging its  spores,  the  heavy  Bpring-Iike  ring  extending  along  the  back  and  over 
the  top.— After  Wossidlo. 


66 


PLANT   STUDIES 


leaf  display,  and   as    a  rule  such   stems   do   not  produce 
many  foliage  leaves,  but  the  leaves  are  apt  to   be  large. 


Fig.  46.    A  common  fern,  showing  the  underground  stem  (rootstock),  which  sends  the 
few  large  foliage  leaves  above  the  surface.— After  Atkinson. 

The  subterranean  position  is  a  good  one,  however,  for 
purposes  of  protection  against  cold  or  drought,  and  when 
the  foliage  leaves  are  killed  new  ones  can  be  put  out  by 


SHOOTS 


57 


the  protected  stem.  This  position  is  also  taken  advantage 
of  for  comparatively  safe  food  storage,  and  sucli  steins  are 
apt  to  become  more  or  less  thickened  and  distorted  by  this 
food  deposit. 

44.  The  procumbent  type. — In  this  case  the  main  body 
of  the  stem  lies  more  or  less  prostrate,  although  the  advanc- 
ing tip  is   usually  erect.     Such  stems  may  spread   in  all 

directions,  and  become  interwoven  into 
mat    or    carpet.       They    are   found 

especially  on  sterile  and  exposed  soil. 


Fig.  47.  A  strawberry  plant,  showing  a  runner  which  has  devel- 
oped a  new  plant,  which  in  turn  has  sent  out  another  run- 
ner.—After  Seubert. 


and  there  may  be  an  important  relation  between  this  fact  and 
their  habit,  as  there  may  not  be  sufficient  building  material 
for  erect  stems,  and  the  erect  position  might  result  in  too 
much  exposure  to  light,  or  heat,  or  wind,  etc.  Whatever 
may  be  the  cause  of  the  procumbent  habit,  it  has  its  advan- 
tages. As  compared  with  the  erect  stem,  there  is  economy 
of  building  material,  for  the  rigid  structures  to  enable  it  to 
stand  upright  are  not  necessary.  On  the  other  hand,  such 
a  stem  loses  in  its  power  to  display  leaves.  Instead  of 
being  free  to  put  out  its  leaves  in  every  direction,  one  side 
is  against  the  ground,  and  the  space  for  leaves  is  diminished 
at  least  one-half.  All  the  leaves  it  bears  are  necessarily 
directed  towards  the  free  side  (see  Fig.  18). 

We  may  be  sure,  however,  that  any  disadvantage  com- 
ing from  this  unfavorable  position  for  leaf  display  is  over- 
balanced by  advantages  in  other  respects.     The  position  is 


58  PLAlsT   STUDIES 

certainly  one  of  protection,  and  it  has  a  further  advantage 
in  the  way  of  migration  and  vegetative  propagation.  As 
the  stem  advances  over  the  ground,  roots  strike  out  of  the 
nodes  into  the  soil.  In  this  way  fresh  anchorage  and  new 
soil  supplies  are  secured  ;  the  old  parts  of  the  stem  may 


'^ 


''^■ 


Fig.  48.  Two  plants  of  a  saxifrage,  showing  rosette  habit,  and  also  the  numerous 
runners  sent  out  from  the  base,  which  strike  root  at  tip  and  produce  new  plants. 
—After  Keener. 

die,  but  the  newer  portions  have  their  soil  connection  and 
continue  to  live.  So  effective  is  this  habit  for  this  kind  of 
propagation  that  plants  with  erect  stems  often  make  use  of 
it,  sending  out  from  near  the  base  special  prostrate  branches, 
which  advance  over  the  ground  and  form  new  plants. 
A  very  familiar  illustration  is  furnished  by  the  straw- 
berry plant,  which  sends  out  peculiar  naked  '^  runners'' 
to  strike  root  and  form  new  plants,  which  then  become 


SHOOTS 


59 


independent  plants  by  the  dying  of  tlie  runners  (see  Figs. 
47,  48). 

45.  The  floating  type. — In  this  case  the  stems  are  sus- 
tained by  water.  Numerous  illustrations  can  be  found  in 
small  inland  lakes  and  slow-moving  streams  (see  Fig.  40). 
Beneath  the  water  these  stems  often  seem  quite  erect,  but 


Fig.  49.    A  submerged  plant  {Ceratophyllum)  with  floating  stems,  showing  the  stem 
joints  bearing  finely  divided  leaves. 

when  taken  out  they  collapse,  lacking  the  buoyant  power 
of  the  water.  Growing  free  and  more  or  less  upright  in 
the  water,  they  seem  to  have  all  the  freedom  of  erect  stems 
in  displaying  foliage  leaves,  and  at  the  same  time  they 
are  not  called  upon  to  build  rigid  structures.  Economy 
of  building  material  and  entire  freedom  to  display  foliage 
would  seem  to  be  a  happy  combination  for  plants.  It  must 
be  noticed,  however,  that  another  very  important  condition 
is  introduced.  To  reach  the  leaf  surfaces  the  light  must 
pass  through  the  water,  and  this  diminishes  its  intensity  so 


60 


PLANT   STUDIES 


greatly  that  the  working  power  of  the  leaves  is  reduced. 
At  no  very  great  depth  of  water  a  limit  is  reached,  beyond 
which  the  light  is  no  longer  able  to  be  of  service  to  the 


leaves  in  their  work. 


Fig.  50.  A  vine  or  liaua  climbing 
the  trunk  of  a  tree.  The  leaves 
are  all  adjusted  to  face  the  light 
and  to  avoid  shading  one  an- 
other as  far  as  possible. 


Hence  it  is  that  water  plants  are 
restricted  to  the  surface  of  the 
water,  or  to  shoal  places  ;  and  in 
such  places  vegetation  is  very 
abundant.  Water  is  so  serious 
an  impediment  to  light  that  very 
many  plants  bring  their  working 
leaves  to  the  surface  and  float 
them,  as  seen  in  water  lilies,  thus 
obtaining  light  of  undiminished 
intensity. 

46.  The  climbing  type. — Climb- 
ing stems  are  developed  especially 
in  the  tropics,  where  the  vegeta- 
tion is  so  dense  and  overshadow- 
ing that  many  stems  have  learned 
to  climb  upon  the  bodies  of  other 
plants,  and  so  spread  their  leaves 
in  better  light  (see  Figs.  50,  55, 
98,  199).  Great  woody  vines 
fairly  interlace  the  vegetation  of 
tropical  forests,  and  are  known 
as  ''lianas,^'  or  "'lianes.'"  The 
same  habit  is  noticeable,  also,  in 
our  temperate  vegetation,  but  it 
is  by  no  means  so  extensively  dis- 
played as  in  the  tropics.  There 
are  a  good  many  forms  of  climb- 
ing stems.  Kemembering  that 
the  habit  refers  to  one  stem  de- 
pending upon  another  for 
mechanical  support,  we  may  in- 
clude many  hedge  plants  in  the 


SHOOTS 


61 


list  of  climbers.  In  this  case  the  stems  are  too  weak  to 
stand  alone,  but  by  interlacing  with  one  another  they  may 
keep  an  upright  position.  There  are  stems,  also,  which 
climb  by  twining  about  their  support,  as  the  hop  vine  and 


Fig.  51.     A  cluster  of  smilax,  showing  the  tendrils  which  enable  it  to  climb,  and  also 
the  prickles.— After  Kernek. 


morning  glory  ;  others  which  put  out  tendrils  to  grasp  the 
support  (see  Figs.  51,  52),  as  the  grapevine  and  star 
cucumber  ;  and  still  others  which  climb  by  sending  out 
suckers  to  act  as  holdfasts,  as  the  woodbine  (see  Figs.  53, 
54),    In  all  these  cases  there  is  an  attempt  to  reach  towards 


62  PLANT   STUDIES 

the  light  without  developing  such  structures  in  the  stem 
as  would  enable  it  to  stand  upright. 

47.  The  erect   type. — This  type   seems   altogether   the 
best  adapted  for  the  proper  display  of  foliage  leaves.   Leaves 


Fig.  52.  Passiou-fiower  vines  climbing  supports  by  means  of  tendrils,  which  may  be 
seen  more  or  less  extended  or  coiled.  The  two  types  of  leaves  upon  a  single  stem 
may  also  be  noted. 

can  be  sent  out  in  all  directions  and  carried  upward  to- 
wards the  light ;  but  it  is  at  the  expense  of  developing  an 
elaborate  mechanical  system  to  enable  the  stem  to  retain 
this  position.  There  is  an  interesting  relation  between 
these  erect  bodies  and  zones  of  temperature.     At  high  alti- 


SHOOTS 


Fig.  53.    Woodbine     ,  >;.>)  in  a  deciduous  forest.    Thf  tne  trunks  are  almost 

covered  by  the  dense  masses  of  woodbine,  whose  leaves  are  adjusted  so  as  to  form 
compact  mosaics,  A  lower  stratum  of  vegetation  is  visible,  composed  of  shrubs 
and  tall  herbs,  showing  that  the  forest  is  somewhat  open. 


tudes  or  latitudes  the  subter- 
ranean and  prostrate  types  of 
foliage-bearing  stems  are  most 
common  ;  and  as  one  passes  to 
lower  altitudes  or  latitudes  the 
erect  stems  become  more  nu- 
merous and  more  lofty.  Among 
stems  of  the  erect  type  the  tree 
is  the  most  impressive,  and  it 
has  developed  into  a  great  vari- 
ety of  forms  or  ^'Miabits/^  Any 
one  recognizes  tlie  great  differ- 
ence in  tlie  habits  of  the  pine 
and  the  elm  (see  Figs.  56, 
57,   58,    59),   and  many  of  our 


Fig.  r>4.  A  portion  of  a  woodbine 
(Ampelopsis).  The  stem  tendrils 
have  attached  themselves  to  a 
smooth  wall  by  means  of  disk-like 
suckers.— After  Strasburger. 


Fig.  55.    A  liana  in  the  Botanic  Garden  at  Peradenyia,  Ceylon.— After  Schimper. 


■V>' 


■.*y 


N  .     .V, 


L 


m 


im& 


Pro.  5*3.  A  tree  of  the  pine  type  Garch),  showing  the  continuous  central  shaft  and 
the  horizontal  branches,  which  tend  to  become  more  upright  towards  the  top  of 
the  tree.  The  general  outline  is  distinctly  conical.  The  larch  is  peculiar  among 
Buch  trees  in  periodically  shedding  its  leaves. 


Fig.  57.  A  pine  tree,  showino;  the  central  shaft  and  also  the  bunching  of  the 
needle  leaves  toward  the  tips  of  the  branches  where  there  is  the  best  exposure 
to  light. 


SHOOTS 


67 


common  trees  may  be  known,  even  at  a  distance,  by  their 
cliaracteristic  habits  (see  Figs.  60,  Gl,  02).  The  difficulty 
of  the  mechanical  problems  solved  by  these  huge  bodies 
is  very  great.  They  maintain  form  and  position  and  en- 
dure tremendous  pressure  and  strain. 


Fig.  58.  An  elm  in  its  winter  condition,  showing  the  al)sence  of  :i  continuons  central 
shaft,  the  main  stem  soon  breaking  up  into  branches,  and  giving  a  spreading  top. 
On  each  side  in  the  background  are  trees  of  the  pine  type,  showing  the  central 
shaft  and  conical  outline. 


68 


PLANT   STUDIES 


48.  Relation  to  light. — As  stems  bearing  foliage  leaves 
hold  a  special  relation  to  light,  it  is  necessary  to  speak  of 
the  influence  of  light  upon  their  direction,  the  response  to 


Fig.  5'J.     Al.  ciLu  u.  loliage,  ahuwun^  die  breaking  up  of  the  truiik  into  uiaucucy  ana 
the  spreading  top. 

which  is  known  as  heliotropism,  already  referred  to  under 
foliage  leaves.  In  the  case  of  an  erect  stem  the  tendency 
is  to  grow  towards  the  source  of  light  (see  Figs.  1,  64). 


SHOOTS  69 

This  has  the  general  result  of  placing  the  leaf  blades  at 
right  angles  to  the  rays  of  liglit,  and  in  this  respect  the 
heliotropism  of  the  stem  aids  in  securing  a  favorable  leaf 
position  (see  Figs.  G3,  G3a).  Prostrate  stems  are  differently 
affected  by  the  light,  however,  being  directed  transversely 
to  the  rays  of  light.     The  same  is  true  of  many  foliage 


Fig.  60.    An  oak  in  its  winter  coiHlitinii.  -  iiraiichinc;.    The  various 

directions  of  the  branches  liave  been  iifierniiiR'a  ny  liie  litrht-relatione. 


brandies,  as  may  be  seen  by  observing  almost  any  tree  in 
wliich  tlie  lower  branches  are  in  the  general  transverse  posi- 
tion. These  branches  generally  tend  to  turn  upwards  when 
they  are  beyond  the  region  of  shading.  Subterranean  stems 
are  also  mostly  horizontal,  but  they  are  out  of  the  influence 
of  light,  and  under  the  influence  of  gravity,  the  response  to 
which  is  known  as  geotrojyism^  which  guides  them  into  tlie 
transverse  position.    The  climbing  stem,  like  the  erect  one, 


70 


PLANT   STUDIES 


Fig.  61.     C'oUuuwuoils.  in  winter  condition,  on  a  sand  (iiiin',  sliuwiuL,'  ilio  brauching 
habit,  and  the  tendency  to  grow  in  groups. 


grows   towards   the   light,   while   floating   stems   may    be 
either  erect  or  transverse. 


B.     Stems  hearing  scale  leaves 

49.  General  character. — A  scale  leaf  is  one  which  does 
not  serve  as  foliage,  as  it  does  not  develop  the  necessary 
chlorophyll.  This  means  that  it  does  not  need  such  an 
exposure  of  surface,  and  hence  scale  leaves  are  usually  much 
smaller,  and  certainly  are  more  inconspicuous  than  foliage 
leaves.  A  good  illustration  of  scale  leaves  is  furnished  by 
the  ordinary  scaly  buds  of  trees,  in  which  the  covering  of 
overlapping  scaly  leaves  is  very  conspicuous  (see  Fig.  65). 
As  there  is  no  development  of  chlorophyll  in  such  leaves. 


SHOOTS 


71 


they  do  not  need  to  be  exposed  to  the  light.  Stems  bearing 
only  scale  leaves,  therefore,  hold  no  necessary  light-relation, 
and  may  be  subterranean  as  well  as  aerial.     For  the  same 


r 

i 

i  ;,|P^^^^mHBBp 

Mm 

BBII?'"^^^^^sl ,  s„, 

^^^m^^mmitmm^'^^^--  ^.v-u^  i^;  ;r  -  ■  :.vf^.;-  .i-^- 

Fig.  62.  A  group  of  weeping  birches,  showing  the  branching  habit  and  the  peculiar 
hanging  branchlets.  The  trunks  also  show  the  habit  of  birch  bark  in  peeling  ofiE 
in  bands  around  the  stem. 


reason  scale  leaves  do  not  need  to  be  separated  from  one 
another,  but  may  overlap,  as  in  the  buds  referred  to. 

Sometimes  scale  leaves  occur  so  intermixed  with  foliage 


Fig,  63.    Sunflowers  with  the  upper  part  of  the  stem  sharply  bent  towards  the  lights 
giving  the  leaves  better  exposure.— After  Schafpkbr. 


SHOOTS 


73 


leaves  that  no  peculiar  stem  type  is  developed.  In  the 
pines  scale  leaves  are  found  abundantly  on  the  stems  which 
are  developed  for  foliage  purposes.  In  fact,  the  main  stem 
axes  of  pines  bear  only  scale  leaves,  while  short  spur-like 
branches  bear  the  characteristic  needles,  or  foliage  leaves, 
but  the  form  of  the 
stem  is  controlled 
by  the  needs  of  the 
foliage.  Some  very 
distinct  types  of 
scale-bearing  stems 
may  be  noted. 

50.  The  bud  type. 
— In  this  case  the 
nodes  bearing  the 
leaves  remain  close 
together,  not  sepa- 
rating, as  is  neces- 
sary in  ordinary 
foliage-bearing 
stems,  and  the 
leaves  overlap.  In 
a  stem  of  this  char- 
acter the  later  joints 
may  become  sepa- 
rated and  bear  foli- 
age leaves,  so  that 
one  finds  scale  leaves 
the  same  stem  axis. 


Fig.  63a.  Cotyledons  of  caetor-oil  bean  ;  the  seedling 
to  the  left  showing  the  ordinary  position  of  the 
cotyledons,  the  one  to  the  right  showing  the  curva- 
ture of  the  stem  in  response  to  light  from  one 
side,— After  Atkinson. 


below  and  foliage  leaves  above  on 
This  is  always  true  in  the  case  of 
branch  buds,  in  wliich  the  scale  leaves  serve  the  purpose 
of  protection,  and  are  aerial,  not  because  they  need  a 
light-relation,  but  because  they  are  protecting  young  foli- 
age leaves  which  do. 

Sometimes  the  scale  leaves  of  this  bud  type  of  stem  do 
not  serve  so  much  for  protection  as  for  food  storage,  and 
become  fleshy.     Ordinary  bulbs,  such  as  those  of  lilies,  etc.. 


74 


PLANT  STUDIES 


Fig.  64,  An  araucarian  pine,  showing  the 
central  shaft,  and  the  regular  clusters  of 
branches  spreading  in  every  direction  and 
bearing  numerous  small  leaves.  The  low- 
ermost branches  extend  downwards  and 
are  the  largest,  while  those  above  become 
more  horizontal  and  smaller.  These  dif- 
ferences in  the  size  and  direction  of  the 
branches  secure  the  largest  light  expo- 
Bore. 


are  of  this  character; 
and  as  the  main  pur- 
pose is  food  storage 
the  most  favorable 
position  is  a  subter- 
ranean one  (see  Fig. 
66).  Sometimes  such 
scale  leaves  become 
very  broad  and  not 
merely  overlap  but  en- 
wrap one  another,  as 
in  the  case  of  the 
onion. 

51.  The  tuber  type. 
— The  ordinary  potato 
may  be  taken  as  an  il- 
lustration (see  Fig. 
67).  The  minute  scale 
leaves,  to  be  found  at 
the  "eyes"  of  the 
potato,  do  not  overlap, 
which  means  that  the 
stem  joints  are  farther 
apart  than  in  the  bud 
type.  The  whole  form 
of  the  stem  results 
from  its  use  as  a  place 
of  food  storage,  and 
hence  such  stems  are 
generally  subterra- 
nean. Food  storage, 
subterranean  position, 
and  reduced  scale 
leaves  are  facts  which 
seem  to  follow  each 
other  naturally. 


SHOOTS 


75 


52.  The  rootstock  type. — This  is  prob- 
ably the  most  common  form  of  subter- 
ranean stem.  It  is  elongated,  as  are  foli- 
age stems,  and  hence  the  scale  leaves 
are  well  separated.  It  is  prominently 
used  for  food  storage,  and  is  also  admirably 
adapted  for  subterranean  migration  (see 
Fig.  68).  It  can  do  for  the  plant,  in  the 
way  of  migration,  what  prostrate  foliage- 
bearing  stems  do,  and  is  in  a  more  protected 
position.  Advancing  beneath  the  ground, 
it  sends  up  a  succession  of  branches 
to  the  surface.  It  is  a  very  efficient 
method  for  the  ^^ spreading^'  of  plants, 
and  is  extensively  used  by  grasses  in  cov- 
ering areas  and  forming  turf.  The  persist- 
ent continuance  of  the  worst  weeds  is  often 
due  to  this  habit  (see  Figs.  69,  70).     It 

is  impossible 


Fig.  G5.  Branch  buds 
of  elm.  Three  buds 
(k)  with  their  over- 
lapping scales  are 
shown,  each  just 
above  the  scar  (b) 
of  an  old  leaf.— 
After  Behrens. 


Fig.  66.  A  bulb,  made  up  of  overlap- 
ping scales,  which  are  fleshy  on 
account  of  food  storage.  —  After 
Gray. 


to    remove 

all    of    the 

indefinitely 

branching 

rootstocks 

from  the  soil, 

and  any  fragments  that  remain 

are  able  to  send  up  fresh  crops 

of  aerial  branches. 

53.  Alternation  of  rest  and 
activity. — In  all  of  the  three 
stem  types  just  mentioned,  it 
is  important  to  note  that  they 
are  associated  with  a  remark- 
able alternation  between  rest 
and  vigorous  activity.  From 
the  branch  buds  the  new  leaves 


76 


PLANT   STUDIES 


emerge  witK 
great  rapidity, 
and  trees  be- 
come covered 
with  new  foliage 
in  a  few  days. 
From  the  sub- 
terranean stems 
the  aerial  parts 
come  up  so 
speedily  that  the 
surface  of  the 
ground  seems  to 
be  covered  suddenly  with  young  vegetation.  This  sudden 
change  from  comparative  rest  to  great  activity  has  been 
well  spoken  of  as  the  ^^  awakening  "  of  vegetation. 


Fig.  67. 


A  potato  plant,  showing  the  subterranean  tubers.— 
After  Strasburger. 


C.     Stems  hearing  floral  leaves 


54.  The  flower. — The  so-called 
^^ flowers''  which  certain  plants 
produce  represent  another  type  of 
shoot,  being  stems  with  peculiar 
leaves.  So  attractive  are  flowers 
that  they  have  been  very  much 
studied ;  and  this  fact  has  led 
many  people  to  believe  that  flowers 
are  the  only  parts  of  plants  worth 
studying.  Aside  from  the  fact 
that  a  great  many  plants  do  not 
produce  flowers,  even  in  those 
that  do  the  flowers  are  connected 
with  only  one  of  the  plant  pro- 
cesses, that  of  reproduction. 
Every  one  knows  that  flowers  are 
exceedingly   variable,  and   names 


Fig.  68.  The  rootstock  of  Solo- 
mon's seal ;  from  the  under  side 
roots  are  developed  ;  and  on  the 
upper  side  are  seen  the  scars 
which  mark  the  positions  of  the 
successive  aerial  branches  which 
bear  the  leaves.  The  advanc- 
ing tip  is  protected  by  scales 
(forming  a  bud),  and  the  posi- 
tions of  previous  buds  are  in- 
dicated by  groups  of  ring-like 
scars  which  mark  the  attach- 
ment of  former  scales.  Advanc- 
ing in  front  and  dying  behind 
such  a  rootstock  may  give  rise 
to  an  indefinite  succession  of 
aerial  plants.— After  Gbat. 


SHOOTS 


77 


have  been  given  to  every  kind  of  variation,  so  that  their 
study  is  often  not  much  more  than  learning  the  definitions 
of  names.  However,  if  we  seek  to  discover  the  life-rela- 
tions of  flowers  we  find  that  they  may  be  stated  very  simply. 
55.  Life-relations. — The  flower  is  to  produce  seed.  It 
must  not  only  put  itself  into  proper  relation  to  do  this,  but 


Fig.  69.  The  rootetock  of  a  rush  (Juncus\  showing  how  it  advances  beneath  the 
ground  and  sends  above  the  surface  a  succession  of  branches.  The  breaking  up 
of  such  a  rootstock  only  results  in  so  many  separate  individuals.— After  Cowles. 


there  must  also  be  some  arrangement  for  putting  the  seeds 
into  proper  conditions  for  developing  new  plants.  In  the 
production  of  seed  it  is  necessary  for  tlie  flower  to  secure  a 
transfer  of  certain  yellowish,  powdery  bodies  which  it  pro- 
duces, known  as  pollen  or  pollen-grains,  to  the  organ  in 
which  the  seeds  are  produced,  known  as  the  pistil.  This 
transfer  is  called  pollination.  One  of  the  important  things, 
therefore,  in  connection  with  tlie   flower,  is  for  it  to  put 


78 


PLANT   STUDIES 


Fig.  70.    An  alpine  willow,  showing  a  strong  rootstock  developing  aerial  branches 
and  roots,  and  capable  of  long  life  and  extensive  migration.— After  Schimper. 


itself  into  such  relations  that  it  may  secure  pollination. 

Besides  pollination, 
which  is  necessary 
to  the  production  of 
seeds,  there  must  be 
an  arrangement  for 
seed  dispersal.  It 
is  always  well  for 
seeds  to  be  scattered, 
so  as  to  be  separated 
from  one  another 
and  from  the  parent 
plant.      The    two 

„  .  ..     .    xt.  r        .    *      srreat  external  prob- 

Fio.  71.    A  flower  of  peony,  showing  the  four  sets  of  °        .     .  -^     . 

floral  organs  :    k,  the  sepals,  together  called  the  IcmS     in     COUnectlOn 

calyx  ;  c,  the  petals,  together  called  the  corolla  ;  with    the     fl  O  W  C  r 

a,  the  numerous  stamens;   g,  the  two  carpels,  ii  j.  77  • 

which  contain  the  ovules.— After  Strasburger,  tnereiore,    are  pOlll' 


SHOOTS 


79 


natio7i  and  seed-dispersal.  It 
is  necessary  to  call  attention 
to  certain  peculiar  features  of 
this  type  of  stem. 

56.  Structures. — The  joints 
of  the  stem  do  not  spread 
apart,  so  that  the  peculiar 
leaves  are  kept  close  together, 
usually  forming  a  rosette-like 
cluster  (see  Fig.  71).  These 
leaves  are  of  four  kinds  :  the 
lowest  (outermost)  ones  (indi- 
vidually sepals,  collectively 
calycc)  mostly  resemble  small 
foliage  leaves  ;  the  next  higher 
(inner)  set  {mdiyidnaWy petals, 
collectively  corolla)  are  usually 
the  most  conspicuous,  delicate 
in  texture  and  brightly  col- 
ored ;  the  third  set  {stamejis) 
produces  the  pollen  ;  the 
highest  (innermost)  set  {car- 
pels) form  the  pistil  and  pro- 
duce the  ovules,  which  are  to 
become  seeds.  These  four  sets 
may  not  all  be  present  in  the 
same  flower  ;  the  members  of 
the  same  set  may  be  more  or 
less  blended  with  one  another, 
forming  tubes,  urns,  etc.  (see 
Figs.  72,  73,  74)  ;  or  the  dif- 
ferent members  may  be  modi- 
fied in  the  greatest  variety  of 
ways. 

Another  peculiarity  of  this 
type  of  stem  is  that  when  the 


Fig.  72.  A  group  of  flowers  of  the  rose 
family.  The  one  at  the  top  (IWen- 
tilla)  shows  three  broad  sepals, 
much  smaller  petals  alternating 
with  them,  a  group  of  stamens,  and 
a  large  receptacle  bearing  numer- 
ous small  carpels.  The  central  one 
(Alchemiila)  shows  the  tips  of  two 
small  sepals,  three  larger  petals 
united  below,  stamens  arising  from 
the  rim  of  the  urn,  and  a  single  pe- 
culiar pistil.  The  lowest  flower  (the 
common  apple)  shows  the  sepals, 
petals,  stamens,  and  three  styles, 
all  arising  from  the  ovary  part  of 
the  pistil.— After  Fockb. 


80 


PLANT  STUDIES 


Fig.  73.  A  flower  of  the  tobacco  plant :  o,  a  complete  flower,  showing  the  calyx  with 
its  sepals  blended  below,  the  funnelform  corolla  made  up  of  united  petals,  and  the 
stamens  just  showing  at  the  mouth  of  the  corolla  tube  ;  6,  acorolla  tube  split  open 
and  showing  the  five  stamens  attached  to  it  near  the  base  ;  c,  a  pistil  made  up  of 
two  blended  carpels,  the  bulbous  base  (containing  the  ovules)  being  the  ovary,  the 
long  stalk-like  portion  the  style,  and  the  knob  at  the  top  the  stigma.— After 
Strase-urger. 

last  set  of  floral  leaves  {carpels)  appear,  the  growth  of  the 
stem  in  length  is  checked  and  the  cluster  of  floral  leaves 


a  b  c  d  e 

Fig.  74,  A  group  of  flower  forms :  a,  a  flower  of  harebell,  showing  a  bell-shaped 
corolla  composed  of  five  petals  ;  6,  a  flow-er  of  phlox,  showing  a  tubular  corolla 
with  its  five  petals  distinct  above  and  sharply  spreading  ;  c,  a  flower  of  dead-nettle, 
showing  an  irregular  corolla  with  its  five  petals  forming  two  lips  above  the  funnel- 
form  base  ;  rf,  a  flower  of  toad-flax,  showing  a  two-lipped  corolla,  and  also  a  spur 
formed  by  the  base  of  the  corolla  ;  e,  a  flower  of  the  snapdragon,  showing  the  two 
lips  of  the  corolla  closed,— After  Gray. 


SHOOTS 


81 


appears  to  be  upon  the  end  of  the  stem  axis.     It  is  usual, 
also,  for  the  short  stem  bearing  the  floral  leaves  to  broaden 


Fig.  75.  The  Star-of -Bethlehem  ( Omithogalu7n\  showing  the  loose  clnster  of  flowers 
at  the  end  of  the  stem.  The  leaves  and  stem  arise  from  a  bulb,  which  produces  a 
cluster  of  roots  below.— After  Strasburgeii. 


at  the  apex  and  form  wliat  is  called  a  receptacle,  upon  which 
the  close  set  floral  leaves  stand. 

Although  many  floral  stems  are  produced  singly,  it  is 


8i4 


PLANT   STUDIES 


very  common  for  them  to  branch,  so  that  the  flowers  appear 
in  clusters,  sometimes  loose  and  spray-like,  sometimes  com- 
pact (see  Figs.  75,   76,  77).     For  example,  the  common 


Fig.  76.    A  flower  cluster  from  a  walnut  tree.— After  Strasburger. 


dandelion  ^'flower"  is  really  a  compact  head  of  flowers. 
All  of  this  branching  has  in  view  better  arrangements  for 
pollination  or  for  seed-distribution,  or  for  both. 

The  subject  of  pollination  and  seed-distribution  will  be 
considered  under  the  head  of  reproduction. 


SHOOTS 


83 


STRUCTURE   AND   FUNCTION   OF   THE   STEM 


57.  Stem  structure. — The  aerial  foliage  stem  is  the  most 
favorable  for  studying  stem  structure,  as  it  is  not  distorted 
hy  its  position  or  by  being  a  depository  for  food.  If  an 
active  twig  of  an  ordinary  woody  plant  be  cut  across,  it  will 


Fig.  77.     Flower  clusters  of  an  umbellifer  {Sium).—MiQT  SxRASBrROER. 

be  seen  that  it  is  made  up  of  four  general  regions  (see  Fig. 
78):  (1)  an  outer  protecting  layer,  which  may  be  stripped 
off  as  a  thin  skin,  the  epidermis  ;  (2)  within  the  epider- 
mis a  zone,  generally  green,  the  cortex  ;  (3)  an  inner  zone 
of  wood  or  vessels,  known  as  the  vascular  reqio7i ;  (4)  a 
central  pith. 

58.  Dicotyledons  and  Conifers. — Sometimes   the  vessels 


84 


TLANT   STUDIES 


Fig.  78.  Section  across  a  young  twig  of  bos 
elder,  showing  the  four  stem  regions  :  e, 
epidermis,  represented  by  the  heavy  bound- 
ing line  ;  c,  cortex  ;  w,  vascular  cylinder  ; 
p,  pith. 


are  arranged  in  a  hollow 
cylinder,  just  inside  of 
the  cortex,  leaving  what 
is  called  pith  in  the 
center  (see  Fig.  78). 
Sometimes  the  pith  dis- 
appears in  older  stems  or 
parts  of  stems  and  leaves 
the  stem  hollow.  "When 
the  vessels  are  arranged 
in  this  way  and  the  stem 
lives  more  than  a  year,  it 
can  increase  in  diameter 
by  adding  new  vessels 
outside   of    the   old.     In 

the  case  of  trees  these  additions  appear  in  cross-section  like 

a  series  of  concentric  rings,  and  as  there  is  usually  but  one 

growth  period  during  the  year,  they  are  often  called  annual 

rings  (see  Fig.  79),  and  the  age  of  a  tree  is  often  estimated 

by    counting    them. 

This  method  of  ascer- 
taining  the   age   of   a 

tree  is  not  absolutely 

certain,   as  there  may 

be    more   than   one 

growth  period  in  some 

years.     In  the  case  of 

trees   and   shrubs   the 

epidermis   is    replaced 

on  the  older  parts  by 

layers  of   cork,  which 

sometimes    becomes 

.^           thick    and    makp«!  Fig.  79.    Section  across  a  twig  of  box  elder  three 

•^  years  old,  showing  three  annual  rings,  or  growth 

up     the     outer    part    of  rings,  in  the  vascular  cylinder.     The  radiating 

what      is      COmmonlv  lines  (m)  which  cross  the  vascular  region  (w)rep- 

^  resent  the  pith  rays,  the  principal  ones  extending 

called   hark.  from  the  pith  to  the  cortex  (c). 


SHOOTS 


85 


Stems  which  increase  in  diameter  mostly  belong  to  the 
great  groups  called  Dicotyledons  and  Conifers.  To  the 
former  belong  most  of  our  common  trees,  such  as  maple, 
oak,  beech,  hickory,  etc,  (see  Figs.  58,  59,  60,  61),  as 
well  as  the  great  majority  of  common  herbs ;  to  the  latter 
belong  the  pines,  hemlocks,  etc.  (see  Figs.  56,  57,  64, 
193,  194).  This  annual  increase  in  diameter  enables  the 
tree  to  put  out  an  increased  number  of  branches  and 
hence  foliage  leaves  each  year,  so 
that  its  capacity  for  leaf  work  be- 
comes greater  year  after  year.  A 
reason  for  this  is  that  the  stem  is 
■conducting  important  food  sup- 
plies to  the  leaves,  and  if  it  in- 
creases in  diameter  it  can  conduct 
more  supplies  each  year  and  give 
work  to  more  leaves. 

59.  Monocotyledons. — In  other 
stems,  however,  the  vessels  are 
arranged  differently  in  the  central 
region.  Instead  of  forming  a  hol- 
low cylinder  enclosing  a  pith,  they 
are  scattered  through  the  central 
region,  as  may  be  seen  in  the  cross- 
section  of  a  corn-stalk    (see  Fig. 

80).  Such  stems  belong  mostly  to  a  great  group  of  plants 
known  as  Monocotyledons,  to  which  belong  palms,  grasses, 
lilies,  etc.  For  the  most  part  such  stems  do  not  increase  in 
diameter,  hence  there  is  no  branching  and  no  increased 
foliage  from  year  to  year.  A  palm  well  illustrates  this 
habit,  with  its  columnar,  unbranching  trunk,  and  its  crown 
of  foliage  leaves,  which  are  about  the  same  in  number  from 
year  to  year  (see  Figs.  81,  82). 

60.  Ferns. — The  same  is  true  of  the  stems  of  most  fern- 
plants,  as  the  vessels  of  the  central  region  are  so  arranged 
that  there  can  be  no  diameter  increase,  though  the  ar- 


;.]" 

f 

1  -■■ 

0 

Fig.  80, 


A  corn-stalk,  showing 
cross-section  and  longitudinal 
section.  The  dots  represent 
the  scattered  bundles  of  ves- 
sels, which  in  the  longitudinal 
section  are  seen  to  be  long 
fiber-like  strands. 


Fig.  81.  A  date  palm,  showing  the  unbranched  columnar  trunk  covered  with  old  leaf 
bases,  and  with  a  cluster  of  huge  active  leaves  at  the  top,  only  the  lowest  portions 
of  which  are  shown.    Two  of  the  very  heavy  fruit  clusters  are  also  shown. 


SHOOTS 


87 


rangement  is  very  different  from  that  found  in  Monocotyle- 
dons. It  will  be  noticed  how  similar  in  general  appearance 
is  the  habit  of  the  tree  fern  and  that  of  the  palm  (see  Fig, 
83). 

Gl.  Lower  plants. — In  the  case  of  moss-plants,  and  such 
algae  and  fungi  as  develop  stems,  the  stems  are  very  much 


Fig.  82. 


A  palm  of  the  palmetto  type  (fan  palm),  with  low  stem  aud  a  crown  of  large 
leaves. 


simpler  in  construction,  but  they  serve  the  same  general 
purpose. 

63.  Conduction  by  the  stem. — Aside  from  the  work  of 
producing  leaves  and  furnishing  mechanical  support,  the 
stem  is  a  great  conducting  region  of  the  plant.  This  sub- 
ject will  be  considered  in  Chapter  X.,  under  the  general 
head  of  **The  Nutrition  of  Plants." 
7 


Pig.  83.  A  group  of  tropical  plants.  To  the  left  of  the  center  is  a  tree  fern,  with  its 
slender  columnar  stem  and  crown  of  large  leaves.  The  large-leaved  plants  to  the 
right  are  bananas  (nmnocotyledons). 


CHAPTER  V 

ROOTS 

63.  General  character. — The  root  is  a  third  prominent 
plant  organ,  and  it  presents  even  a  greater  variety  of  rela- 
tions than  leaf  or  stem.  In  whatever  relation  it  is  found 
it  is  either  an  absorbent  organ  or  a  holdfast,  and  very  often 
both.  For  such  work  no  light-relation  is  necessary,  as  in 
the  case  of  foliage  leaves  ;  and  there  is  no  leaf-relation,  as 
in  the  case  of  stems.  Roots  related  to  the  soil  may  be 
taken  as  an  illustration. 

It  is  evident  that  a  soil  root  anchors  the  plant  in  the 
soil,  and  also  absorbs  water  from  the  soil.  If  absorption  is 
considered,  it  is  further  evident  that  the  amount  of  it  will 
depend  in  some  measure  upon  the  amount  of  surface  which 
the  roots  expose  to  the  soil.  We  have  already  noticed  that 
the  foliage  leaf  has  the  same  problem  of  exposure,  and  it 
solves  it  by  becoming  an  expanded  organ.  The  question 
may  be  fairly  asked,  therefore,  why  are  not  roots  expanded 
organs  ?  The  receiving  of  rays  of  light,  and  the  absorbing 
of  water  are  very  diiferent  in  their  demands.  In  the  former 
case  a  flat  surface  is  demanded,  in  the  latter  tubular  pro- 
cesses. The  increase  of  surface  in  the  root,  therefore,  is 
obtained  not  by  expanding  the  organ,  but  by  multiplying 
it.  Besides,  to  obtain  the  soil  water  the  roots  must  burrow 
in  every  direction,  and  must  send  out  their  delicate  thread- 
like branches  to  come  in  contact  with  as  much  soil  as  pos- 
sible. Furthermore,  in  soil  roots  absorption  is  not  the  only 
thing  to  consider,  for  the  roots  act  as  holdfasts  and  must 
grapple  the  soil.     This  is  certainly  done  far  more  effectively 

8P 


90 


PLANT   STUDIES 


by  numerous  thread-like  processes  spreading  in  every  direc- 
tion than  by  flat,  expanded  processes. 

It  should  also  be  noted  that  as  soil  roots  are  subterra- 
nean they  are  used  often  for  the  storage  of  food,  as  in  the 
case  of  many  subterranean  stems.  Certain  prominent  root 
types  may  be  noted  as  follows  : 

64.  Soil  roots.— These  roots  push  into  the  ground  with 

great  energy, 
and  their  ab- 
sorbing sur- 
faces are  en- 
tirely covered. 
Only  the  young- 
est parts  of  a 
root  system 
absorb  actively, 
the  older  parts 
transporting 
the  absorbed 
material  to  the 
stem,  and  help- 
ing to  grip  the 
soil.  The  soil 
root  is  the  most 
common  root 
type,  b  eing 
used  by  the  great  majority  of  seed  plants  and  fern  plants, 
and  among  the  moss  plants  the  very  simple  root-like  pro- 
cesses are  mostly  soil-related.  To  such  roots  the  water  of 
the  soil  presents  itself  either  as/ree  water — that  is,  water 
that  can  be  drained  away — or  as  films  of  water  adhering  to 
each  soil  particle,  often  called  water  of  adhesion.  To  come 
in  contact  with  this  water,  not  only  does  the  root  system 
usually  branch  profusely  in  every  direction,  but  the  youngest 
branches  develop  abundant  absorbing  hairs,  or  root  hairs 
(see  Fig.  84),  which  crowd  in  among  the  soil  particles  and 


Fig.  84  Root  tips  of  corn,  showing  root  hairs  and  their 
position  in  reference  to  the  growing  tip :  1,  in  soil  (higher 
up  the  hairs  become  much  more  abundant  and  longer) ; 
2,  in  moist  air. 


ROOTS. 


91 


absorb  moisture  from  them. 


Fig.  85.  Apparatus  to  show  the  influence 
of  water  (hydrotropism)  upon  the  direc- 
tion of  roots.  The  ends  {a)  of  the  box 
have  hooks  for  hanging,  while  the  box 
proper  is  a  cylinder  or  trough  of  wire 
netting  and  is  filled  with  damp  sawdust. 
In  the  sawdust  are  planted  peas  {g), 
whose  roots  {h.  i,  k,  m)  first  descend  until 
they  emerge  from  the  damp  sawdust,  but 
soon  turn  back  toward  it.— After  Sachs. 


By  these  root  hairs  the  ab- 
sorbing surface,  and  hence 
the  amount  of  absorption, 
is  greatly  increased.  Indi- 
vidual root  hairs  do  not  last 
very  long,  but  new  ones  are 
constantly  appearing  just 
behind  the  advancing  root 
tips,  and  the  old  ones  are 
as  constantly  disappearing. 
(1)  Geotropism  and  hy- 
drotropism.— Many  outside 
influences  affect  roots  in 
the  direction  of  their 
growth,  and  as  soil  roots 
are  especially  favorable  for 
observing  these  influences, 
two  prominent  ones  may 
be  mentioned.  The  influ- 
ence of  gravity,  or  the  earth 
influence,  is  very  strong 
in  directing  the  soil   root. 


Fig.  86. 


A  raspberry  plant,  whose  stem  has  been  bent  down  to  the  soil  and  has 
"struck  root."— After  Beal. 


92 


PLANT  STUDIES 


As  is  well  known,  when  a  seed  germinates  the  tip  that  is  to 
develop  the  root  turns  towards  the  earth,  even  if  it  has 
come  from  the  seed  in  some  other  direction.  This  response 
to  gravity  by  the  plant  is  known  as  geotropisin.  Another 
directing  influence  is  moisture,  the  response  to  which  is 


Fig.  87.  A  section  through  the  leaf-stalk  of  a  yellow  pond-lily  {Nuphar),  showing  the 
numerous  conspicuous  air  passages  {s)  by  means  of  which  the  parts  under  water 
are  aerated ;  h,  internal  hairs  projecting  into  the  air  passages ;  v,  the  much 
reduced  and  comparatively  few  vascular  bundles. 

known  as  hydrotrojnsm.  By  means  of  this  the  root  is  di- 
rected towards  the  most  favorable  water  supply  in  the  soil. 
Ordinarily,  geotropism  and  hydrotropism  direct  the  root 
in  the  same  general  way,  and  so  reinforce  each  other ;  but 
the  following  experiment  may  be  arranged,  which  will 
separate  these  two  influences.  Bore  several  small  holes  in 
the  bottom  of  a  box,  suspended  as  indicated  in  Fig.  85, 
and  cover  the  bottom  and  surround  the  box  with  blotting 
paper.     Pass  the   root  tips  of  several  germinated  seeds 


ROOTS 


93 


through  the  holes,  so  that  the  seeds  rest  on  the  paper,  and 
the  root  tips  hang  through  the  holes.  If  the  paper  is  kept 
moist  germination  will  continue,  but  geotropism  will  direct 
the  root  tips  downwards  and  hydrotropism  (response  to 
the  moist  paper)  will  direct  them  upwards.  In  this  way 
they  will  pursue  a  devious  course,  now  directed  by  one 
influence  and  now  by  the  other. 

If  a  root  system  be  examined  it  will  be  found  that  when 
there  is  a  main  axis  (tap 
root)  it  is  directed 
steadily  downwards, 
while  the  branches  are 
directed  differently. 
This  indicates  that  all 
parts  of  a  root  system 
are  not  alike  in  their 
response  to  these  influ- 
ences. Several  other 
influences  are  also  con- 
cerned in  directing  soil 
roots,  and  the  path  of 
any  root  branch  is  a 
result  of  all  of  them. 
How  variable  they  are 
may  be  seen  by  the 
numerous  directions  in 
which    the    branches 

travel,  and  the  whole  root  system  preserves  the  record  of 
these  numerous  paths. 

(2)  The  pull  071  the  stem. — Another  root  property  may 
be  noted  in  connection  with  the  soil  root,  namely  the  pull 
on  the  stem.  When  a  strawberry  runner  strikes  root  at 
tip  (see  Fig.  47),  the  roots,  after  they  obtain  anchorage  in 
the  soil,  pull  the  tip  a  little  beneath  the  surface,  as  if  they 
had  gripped  the  soil  and  then  slightly  contracted.  The 
same  thing    may  be   observed    in  the   process  known  as 


Fig.  88.  A  section  through  the  stem  of  a  water- 
wort  (Elatine),  showing  the  remarkably  large 
and  regularly  arranged  air  passages  for  root 
aeration.  The  single  reduced  vascular  bundle 
is  central  and  connected  with  the  small  cor- 
tex by  thin  plates  of  cells  which  radiate  like 
the  spokes  of  a  wheel.— After  Schenck. 


94 


PLANT   STUDIES 


'"layering,"  by  which  a  stem,  as  a  bramble,  is  bent  down 
and  covered  with  soil.  The  covered  joints  strike  root,  and 
the  pulling  follows  (see  Fig.  86).  A  very  plain  illustration 
of  this  pulling  by  roots  can  be  obtained  from  many  tuberous 
plants.  Tubers,  bulbs,  rootstocks,  etc.,  are  underground 
structures  which  have  been  observed  to  bury  themselves 
deeper  and  deeper  in  the  soil.    This  is  effected  by  the  young 


Pig.  89.    Section  through  the  leaf  of  a  quillwort  (Isoetes),  showing  the  four  large  air 
chambers  (a),  the  central  vascular  region  (b),  and  the  very  poorly  developed  cortex. 

roots  which  they  continue  to  put  forth.  These  roots  grip 
the  soil,  then  contract,  and  the  tuber  is  pulled  a  little  deeper. 
The  compact  tuber  known  as  the  Indian  turnip  ('^  Jack-in- 
the-pulpit ")  has  been  found  to  bury  itself  very  deeply  and 
rapidly,  and  this  may  be  observed  by  transplanting  a  young 
and  vigorous  tuber  into  a  pot  of  loose  soil. 

(3)  Soil  dangers. — In  this  connection  certain  soil  dan- 
gers and  the  response  of  the  roots  should  be  noted.  The 
soil  may  become  poor  in  water  or  poor  in  certain  essential 
materials,  and  this  results  in  an  extension  of  the  root  sys- 


KOOTS 


95 


tern,  as  if  seeking  for  water  and  the  essential  materials. 
Sometimes  tlie  root  system  becomes  remarkably  extensive, 
visiting  a  large  amount  of  soil  in  order  to  procure  the 
necessary  supplies.  Sometimes  the  soil  is  poor  in  heat,  and 
root  activity  is  interfered  with.  In  such  cases  it  is  very 
common  to  find  the  leaves 
massed  against  the  soil,  thus 
slightly  checking  the  loss  of 
heat. 

Most  soil  roots  also  need  free 
air,  and  when  water  covers  the 
soil  the  supply  is  cut  off.  In 
many  cases  there  is  some  way 
by  which  a  supply  of  free  air 
may  be  brought  down  into  the 
roots  from  the  parts  above 
water  ;  sometimes  by  large  air 
passages  in  leaves  and  stems 
(see  Figs.  87,  88,  89,  90) ;  some- 
times by  developing  special  root 
structures  which  rise  above  the 
water  level,  as  prominently 
shown  by  the  cypress  in  the 
development  of  hiees.  These 
knees  are  outgrowths  from  roots 
beneath  the  water  of  the  cypress 
swamp,  and  rise  above  the  water  level,  thus  reaching  the 
air  and  aerating  the  root  system  (see  Fig.  91).  It  has  been 
shown  that  if  the  water  rises  so  high  as  to  flood  the  knees 
for  any  length  of  time  the  trees  will  die,  but  it  does  not 
follow  that  this  is  the  chief  reason  for  their  development. 

65.  Water  roots.— A  very  different  type  of  root  is  devel- 
oped if  it  is  exposed  to  free  water,  without  any  soil  relation. 
If  a  stem  is  floating,  clusters  of  whitish  thread-like  roots 
usually  put  out  from  it  and  dangle  in  the  water.  If  tlie  water 
level  sinks  so  as  to  bring  the  tips  of  these  roots  to  the  mucky 


Fig.  90.  Longitudinal  section 
through  a  young  quillwort  leaf, 
showing  that  the  four  air  cham- 
bers shown  in  Fig.  89  are  not  con- 
tinuous passages,  but  that  there 
are  four  vertical  rows  of  promi- 
nent chambers.  The  plates  of 
cells  separating  the  chambers  in 
a  vertical  row  very  soon  become 
dead  and  full  of  air.  In  addition 
to  the  work  of  aeration  these  air 
chambers  are  very  serviceable  in 
enabling  the  leaves  to  float  when 
they  break  off  and  carry  the  com- 
paratively heavy  spore  cases. 


ROOTS 


97 


Fig.  Oe, 


'^Anthurium),  BhowiDg  its  large  leaves,  and  bunches  of 
aerial  roots. 


soil  they  usually  do  not  penetrate  or  enter  into  any  soil  re- 
lation. Such  pure  water  roots  may  be  found  dangling  from 
the  under  surface  of  the  common  duck  weeds,  which  often 
cover  the  surface  of  stagnant  water  with  their  minute, 
green,  disk-like  bodies. 


98 


PLANT   STUDIES 


Plants  which  ordinarily  develop  soil  roots,  if  brought 
into  proper  water  relations,  may  develop  water  roots.  For 
instance,  willows  or  other  stream  bank  plants  may  be  so 
close  to  the  water  that  some  of  the  root  system  enters  it. 
In  such  cases  the  numerous  clustered  roots  show  their  water 


Fig.  93.    An  orchid,  showing  aerial  roots. 

character.  Sometimes  root  systems  developing  in  the  soil 
may  enter  tile  drains,  when  water  roots  will  develop  in  such 
clusters  as  to  choke  the  drain.  The  same  bunching  of  water 
roots  may  be  noticed  when  a  hyacinth  bulb  is  grown  in  a 
vessel  of  water. 

66.  Air  roots. — In  certain  parts  of  the  tropics  the  air  is 
so  moist  that  it  is  possible  for  some  plants  to  obtain  suffi- 


ROOTS 


99 


cient  moisture  from  this  source,  without  any  soil-relation 
or  water-relation.  Among  these  plants  the  orchids  are 
most  notable,  and  they  may  be  observed  in  almost  any 
greenhouse.  Clinging  to  the  trunks  of  trees,  usually  imi- 
tated in  the  greenhouse  by  nests  of  sticks,  they  send  out 
long  roots  which  dangle  in  the  moist  air  (see  Figs.  93,  94). 
It  is  necessary  to  have  some  special  absorbing  arrange- 
ment, and  in  the  orchids  this 
is  usually  provided  by  the  de- 
velopment of  a  sponge-like 
tissue  about  the  root  known 
as  the  rekfmen,  which  greed- 
ily absorbs  the  dew  or  water 
trickling  down  the  plant.  See 
also  Figs.  92,  95,  96,  97. 

67.  Clinging  roots. — These 
roots  are  developed  to  fasten 
the  plant  body  to  some  sup- 
port, and*  do  no  work  of  ab- 
sorption (see  Fig.  98).  Very 
common  illustrations  may  be 
obtained  from  the  ivies,  the 
trumpet  creeper,  etc.  These 
roots  cling  to  various  supports, 
stone  walls,  tree  trunks,  etc., 
by  sending  minute  tendril- 
like branches  into  the  crevices.  The  sea-weeds  (algfp) 
develop  grasping  structures  extensively,  a  large  majority 
of  them  being  anchored  to  rocks  or  to  some  rigid  support 
beneath  the  water,  and  their  bodies  floating  free.  The 
root-like  processes  by  which  this  anchorage  is  secured  are 
very  prominent  in  many  of  the  common  marine  sea-weeds 
(see  Fig.  162). 

68.  Prop  roots. — Some  roots  are  developed  to  prop 
stems  or  wide-spreading  branches.  In  swampy  ground,  or 
in  tropical  forests,  it  is  very  common  to  find  the  base  of 


Fig.  94.      An  orchid,  showing  aerial 
roots  and  thick  leaves. 


Pig.  95.    Aetaghorn  fern  (Platycerium),  an  aerial  plant  of  the  tropics.    About  it  is  a 
vine,  which  sh-^ue  \hc  'oaves  adjiiftcil  to  theliijhted  side. 


FiQ.  97.    Live  oaks,  in  the  Gulf  States,  upon  which  are  growing  masses  of  long  moss 
or  blacli  moss  ( TiUandsia),  a  common  aerial  plant 


Fig.  98.     A  tropical  forest,  showing  the  cord-likf  holdfasts  developed  by  an  epiphyte, 
which  pass  around  the  tree  trunks  like  tightly  bound  ropes.— After  Kerner. 


ROOTS 


103 


tree  trunks  buttressed  by  such  roots  which  extend  out  over 
and  beneath  the  surface,  and  divide  the  area  about  the  tree 
into  a  series  of  irregular  chambers  (see  Fig.  100).      Some- 


FlQ.  99.     A  bLrcw-pinc  yi'diniiunis),  from  the    Indian   i)ceiin    ngion, 
prominent  prop  roots  put  out  near  the  base. 


stiowing  the 


times  a  stem,  either  inclined  or  with  a  poorly  developed 
primary  root  system,  puts  out  prop  roots  which  support 
it,  as  in  the  screw-pine   (see  Fig.  99).     A  notable  case  is 


106 


PLANT   STUDIES 


that  of  the  banyan  tree,  whose  wide-spreading  branches 
are  supported  by  prop  roots,  which  are  sometimes  very 
numerous  (see  Fig.   101).     The  immense  banyans   usually 

illustrated  are 
especially  culti- 
vated as  sacred 
trees,  the  prop 
roots  being  as- 
sisted in  pene- 
trating the  soil. 
There  is  record 
of  such  a  tree  in 
Ceylon  with  350 
large  and  3,000 
small  prop  roots, 
able  to  cover  a 
village  of  100 
huts. 

69.  Parasites. 
— Besides  the 
roots  mentioned 
above,  certain 
plants  develop 
root-like  pro- 
cesses which  re- 
late them  to  hosts. 
A  host  is  a  liv- 
ing plant  or 
animal  upon 
which  some 
other  plant  or 
animal  is  living 
as  a  parasite. 
The  parasite  gets  its  supj^lies  from  the  host,  and  must  be 
related  to  it  properly.  If  the  parasite  grows  upon  the 
surface  of  its  host,  it  must  penetrate  the  body  to  obtain 


Fig.  102.  A  dodder  plant  parasitic  on  a  willow  twig.  The 
leafless  dodder  twines  about  the  willow,  and  sends  out 
sucking  processes  which  penetrate  and  absorb.— After 
Strasburger. 


ROOTS 


107 


food  supplies. 
Therefore,  pro- 
cesses are  devel- 
oped which  pene- 
trate and  absorb. 
The  mistletoe  and 
dodder  are  seed- 
plants  which  have 
this  habit,  and 
both  have  such 
processes  (see  Figs. 
102,  103).  This 
habit  is  much  more 
extensively  devel- 
oped, however,  in 
a  low  group  of 
plants  known  as 
the  fungi.  Many 
of  these  parasitic 
fungi  live  upon 
plants  and  animals, 
common  illustrations  being  the  mildews  of  lilac  leaves  and 
many  other  plants,  the  rust  of  wheat,  the  smut  of  corn,  etc. 

70.  Root  structure. 
— In  the  lowest  groups 
of  plants  (alga^,  fungi, 
and  moss-plants)  true 
roots  are  not  formed, 
but  very  simple  struc- 
tures, generally  luiir- 
like  (see  Fig.  104).  In 
fern-plants  and  seed- 
plants,  however,  the 
root  is  a  complex 
structure,  so  different 
from  the  root-like  pro- 


FiG.  103.  A  section  showing  the  living  connection 
between  dodder  and  a  golden  rod  upon  which  it  is 
growing.  The  penetrating  and  absorbing  organ  (h) 
has  passed  through  the  cortex  (c),  the  vascular 
zone  (6),  and  is  disorganizing  the  pith  Qj). 


Fig.  104.  Section  through  the  thallus  of  a  liver- 
wort (Marchantia),  showing  the  halr-likc  pro- 
cesses (rhizoids)  which  come  from  the  under 
surface  and  act  as  roots  in  gripping  and  ab- 
sorbing. In  the  epidermis  of  the  upper  surface 
a  chimney-like  ojjening  is  seen,  leading  into 
a  chamber  containing  cells  with  chloroplaste. 


108 


PLANT  STUDIES 


cesses  of  the  lower  groups  that  it  is  regarded  as  the  only 
true  root.     It  is  quite  uniform  in  structure,  consisting  of  a 

tough  and  fibrous 
central  axis  surround- 
ed by  a  spongy  region 
(Fig.  105).  The 
tough  axis  is  most- 
ly made  up  of  ves- 
sels, so  called  be- 
cause they  conduct 
material,  and  is  called 
the  vascular  axis. 
The  outer  more 
spongy  region  is  the 
cortex,  which  covers 
the  vascular  axis  like 
a  thick  skin. 

One  of  the  pecu- 
liarities of  the  root  is 
that  the  branches 
come  from  the  vascu- 
lar axis  and  burrow 
through  the  cortex, 
so  that  when  the  lat- 
ter is  peeled  off  the 
branches  are  left  at- 
tached to  the  axis, 
and  the  cortex  shows 
the  holes  through 
which  they  passed. 

Another  pecu- 
liarity of  the  root  is 
that  it  elongates  only  by  growth  at  the  tip,  and  in  the  soil 
this  delicate  growing  tip  is  protected  by  a  little  cap  of  cells, 
known  as  the  roof-cap. 


Fig.  105.  A  longitudinal  section  through  the  root 
tip  of  spiderwort,  showing  the  central  vascular 
axis  (2)1),  surrounded  by  the  cortex  (p),  outside 
of  the  cortex  the  epidermis  (e)  which  disappears 
in  the  older  parts  of  the  root,  and  the  promi- 
nent root-cap  (c). 


CHAPTER  VI 

REPRODUCTIVE  ORGANS 


It  will  be  remembered  that  nutrition  and  reproduction 
are  the  two  great  functions  of  plants.  In  discussing 
foliage  leaves,  stems,  and  roots,  they  were  used  as  illustra- 
tions of  nutritive  organs,  so  far  as  their  external  relations 
are  concerned.  We  shall  now 
briefly  study  the  reproductive 
organs  from  the  same  point 
of  view,  not  describing  the 
processes  of  reproduction,  but 
some  of  the  external  relations. 

71.  Vegetative  multiplica- 
tion.— Among  the  very  lowest 
plants  no  special  organs  of 
reproduction  are  developed, 
but  most  plants  have  them. 
There  is  a  kind  of  reproduc- 
tion by  which  a  portion  of 
the  parent  body  is  set  apart  to 
produce  a  new  plant,  as  when 
a  strawberry  runner  produces 
a  new  strawberry  plant,  or 
when  a  willow  twig  or  a  grape 
cutting  is  planted  and  produces  new  plants,  or  when  a  potato 
tuber  (a  subterranean  stem)  produces  new  potato  plants,  or 
when  pieces  of  Begonia  leaves  are  used  to  start  new  Begonias. 
This  is  known  as  vegetative  multipUcatio)i,  a  kind  of  rej^ro- 
duction  which  does  not  use  special  reproductive  organs. 

109 


Fig.  106.  A  group  of  spores :  J., 
spores  from  a  common  mold  (a 
fungus),  which  are  so  minute  and 
light  that  they  are  carried  about  by 
the  air  ;  B,  two  spores  from  a  com- 
mon alga  {Llothrix),  which  can 
swim  by  means  of  the  hair-like 
processes;  C,  the  conspicuous  dotted 
cell  is  a  spore  developed  by  a  com- 
mon mildew  (a  fungus),  which  is 
carried  about  by  currents  of  air. 


liu 


PLANT   STUDIES 


72.  Spore  reproduction.— Besides  vegetative  multiplica- 
tion most  plants  develop  special  reproductive  bodies, 
known  as  spores,  and  this  kind  of  reproduction  is  known 
as  spore  reproduction.  These  spores  are  very  simple 
bodies,  but  have  the  power  of  producing  new  individuals. 
There  are  two  great  groups  of  spores,  differing  from  each 
other  not  at  all  in  their  powers,  but  in  the  method  of  their 
production   by  the   parent   plant.     One  kind  of  spore  is 

produced  by  dividing 
certain  organs  of  the 
parent ;  in  the  other 
case  two  special  bodies 
of  the  parent  blend 
together  to  form  the 
spore.  Although  they 
are  both  spores,  for 
convenience  we  may 
call  the  first  kind 
spores  (see  Figs.  106, 
109),  and  the  second 
kind  eggs  (see  Fig. 
107).*  The  two  special 
bodies  which  blend  to- 
gether to  form  an  Qgg 
are  called  gametes  (see 


Fig.  107.  Fragments  of  a  common  alga  {Spi- 
rogyra).  Portions  of  two  threads  are  shown, 
which  have  been  joined  together  by  the  grow- 
ing of  connecting  tubes.  In  the  upper  thread 
four  cells  are  shown,  three  of  which  contain 
eggs  (2),  while  the  cell  marked  g,  and  its  mate 
of  the  other  thread  each  contain  a  gamete, 
the  lower  one  of  which  will  pass  through  the 
tube,  blend  with  the  upper  one,  and  form 
another  egg. 


Figs.  107,  108,  109).  These  terms  are  necessary  to  any 
discussion  of  the  external  relations.  Most  plants  develop 
both  spores  and  eggs,  but  they  are  not  always  equally  con- 
spicuous. Among  the  algae,  both  spores  and  eggs  are  prom- 
inent ;  among  certain  fungi  the  same  is  true,  but  many 
fungi  are  not  known  to  produce  eggs  ;  among  moss-plants 
the  spores  are  prominent  and  abundant,  but  the  egg  is 
concealed  and  not  generally  noticed.     What  has  been  said 

*  It  is  recognized  that  this  spore  is  really  a  fertilized  egg,  but  in 
the  absence  of  any  accurate  simple  word,  the  term  egg  is  used  for  con- 
venience. 


REPRODUCTIVE   ORGANS 


111 


of  the  moss-plants  is  still  more  true 
of  the  fern-plants ;  while  among 
the  seed-plants  certain  spores  (pol- 
len grams)  are  conspicuous  (see 
Fig.  110),  but  the  eggs  can  be  ob- 
served only  by  special  manipulation 
in  the  laboratory.  Seeds  are  neither 
spores  nor  eggs,  but  peculiar  repro- 
ductive bodies  which  the  hidden 
egg  has  helped  to  produce. 

73.  Germination.  —  Spores  and 
eggs  are  expected  to  germinate ; 
that  is,  to  begin  the  development 
of  a  new  plant.  This  germination 
needs  certain  external  conditions, 
prominent  among  which  are  defi- 
nite amounts  of  heat,  moisture, 
and  oxygen,  and  sometimes  light. 
Conditions  of  germination  may  be 
observed  most  easily  in  connection 
with  seeds.  It  must  be  understood, 
however,  that  what  is  called  the 
germination  of  seeds  is  something 

very  different  from  the  germination 
of  spores  and  eggs.  In  the  latter 
cases,  germination  includes  the  very 
beginnings  of  the  young  plant.  In 
the  case  of  a  seed,  germination  begun 
by  an  egg  has  been  checked,  and 
seed  germination  is  its  renewal.  In 
other  words,  an  egg  has  germinated 
and  produced  a  young  plant  called 
the  *^ embryo/'  and  the  germination 
of  the  seed  simply  consists  in  the 
continued  growth  and  the  escape  of 
this  embryo. 


Fig.  108.  A  portion  of  the 
body  of  a  common  alga 
( (Edogonium),  showing 
gametes  of  very  unequal  size 
and  activity  ;  a  very  large 
one  id)  is  lying  in  a  globular 
cell,  and  a  very  small  one  ia 
entering  the  cell,  another 
similar  one  («)  being  just 
outside.  The  two  small 
gametes  have  hair-like  pro- 
cesses and  can  swim  freely. 
The  small  and  large  gam- 
etes unite  and  form  an  egg. 


Fig.  109.  A  group  of  swim- 
ming cells  :  .4,  a  spore  of 
CEdogonium  (an  alga) ; 
.B,  spores  of  Ulothrix  (an 
alga) ;  C,  a  gamete  of 
Equisetum  (horse-tail  or 
Bcouriug  rush). 


112 


PLANT   STUDIES 


Pig.  110.  A  pollen  grain  (spore)  from  the 
pine,  which  develops  wings  (iv)  to  assist 
in  its  transportation  by  currents  of  air. 


It  is  evident  that  for 
the  germination  of  seeds 
light  is  not  an  essential 
condition,  for  they  may 
germinate  in  the  light  or 
in  the  dark  ;  but  the  need 
of  heat,  moisture,  and 
oxygen  is  very  apparent. 
The  amount  of  heat  re- 
quired for  germination 
varies  widely  with  different 
seeds,  some  germinating 
at  much  lower  tempera- 
tures than  others.     Every 

kind  of  seed,  or  spore,  or  egg  has  a  special  temperature 

range,  below  which  and  above  which 

it  cannot  germinate.     The  two  limits 

of    the    range    may    be    called    the 

lowest   and   highest   points,   but   be- 
tween the  two  there  is  a  best  point 

of  temperature  for  germination.    The 

same  general  fact  is  true  in  reference 

to  the  moisture  supply. 

74.  Dispersal  of  reproductive  bodies. 

— Among  the  most  striking  external 

relations,    however,    are    those    con- 
nected with  the  dispersal  of  spores, 

gametes,  and  seeds.     Spores  and 

seeds  must  be  carried  away  from  the 

parent    plant,    and    separated    from 

each    other,    out    of    the    reach    of 

rivalry   for   nutritive    material ;   and 

gametes    must    come    together    and     ^     ,,,    ,      ,   ^, 

*  ^  ,  Fig.  111.   A  pod  of  fireweed 

blend  to  form  the  eggs.     Conspicuous        (EpUobium)  opening  and 
among  the  means  of  transfer  are  the        exposing  its  plumed  seeds 

.  which  are  transported  by 

following.  the  wind.-After  Beal. 


REPRODUCTIVE   ORGANS 


113 


75.  Dispersal  by  locomotion.— The   common  method  of 
locomotion  is  by  means  of  movuble  hairs  {cilia)  developed 
upon  the  reproductive  body,  which  propel  it  through  the 
water   (see  Fig.   109). 
Swimming  spores  are 
very   common   among 
the  algas,  and  at  least 
one    of    the    gametes 
in  alga?,   moss-plants, 
and    fern-plants     has 
the    power   of    swim- 
ming   by   means    of 
cilia. 

7G.  Dispersal  by 
water.  —  It  is  very 
common  for  repro- 
ductive bodies  to  be 
transported  by  cur- 
rents of  water.  The 
spores  of  many  water 
plants  of  all  groups, 
not  constructed  for 
locomotion,  are  thus 
floated  about.  This 
method  of  transfer  is 
also  ver}^  common 
among  seeds.  Many 
seeds  are  buoyant,  or 
become  so  after  soak- 
ing in  water,  and 
may  be  carried  to 
great  distances  by 
currents.  For  this  reason  tlie  plants  growing  upon  the 
banks  or  flood-plains  of  streams  may  have  come  from  a 
wide  area.  Many  seeds  can  even  endure  prolonged  soak- 
ing in  sea-water,  and  tlien  germinate.     Darwin  estimated 


Fig.  11:2.  The  upper  figure  to  the  left  is  ar  opening 
pod  of  fireweed  discharging  its  phinied  seeds. 
The  lower  figure  represents  the  seed-like  fruits 
of  Clematis  with  their  long  tail-like  plumes.— 
After  Kerner. 


114 


PLANT   STUDIES 


that  at  least 
fourteen  per 
cent,  of  the 
seeds  of  any 
country  can  re- 
tain their  vital- 
ity in  sea-water 
for  twenty- 
eight  clays.  At 
the  ordinary 
rate  of  move- 
ment of  ocean 
currents,  this 
length  of  time 
would  permit 
such  seeds  to 
be  transported 
over  a  thou- 
sand miles, 
thus  making 
possible   a   very   great   range   in   distribution. 

77.  Dispersal  of  spores  by  air. — This  is  one  of  the  most 
common  methods  of  transport- 
ing spores  and  seeds.  In  most 
cases  spores  are  sufficiently 
small  and  light  to  be  trans- 
ported by  the  gentlest  move- 
ments of  air.  Among  the 
fungi  this  is  a  very  common 
method  of  spore  dispersal  (see 
Fig.  106),  and  it  is  extensively 
used  in  scattering  the  spores 
of  moss-plants,  fern-plants  (see 
Fig.  45),  and  seed-plants. 
Among  seed-plants  this  is  one     fi«- "^    seed-iike  fruits  oi  senecio 

°  T 1  •  .  ^^'^^^  plumes  for  dispersal  by  air.— 

method  of  pollination,  the        After kerner. 


Fig.  113.  A  ripe  dandelion  head,  showing  the  mass  of 
plumes,  a  few  seed-like  fruits  with  their  plumes  still 
attached  to  the  receptacle,  and  two  fallen  off.— After 
Kerner. 


REPRODUCTIVE   ORGANS 


115 


Fig.  115.    A  winged  seed  of  Bignonia.— After  Strasburger. 


spores  called  pollen 
and  occasionally 
falling  ujDon  the 
right  spot  for 
germination. 
With  such  an 
agent  of  transfer 
the  pollen  must 
be  very  light  and 
powdery,  and 
also  very  abun- 
dant, for  it  must 
come  down  al- 
most like  rain  to  be 


grains  being  scattered  by  the  wind. 


Fig.  117. 


Winged    fruit   of 
Kerner. 


Fig.  116.    Winged  fruit  of  maple.— After  Kerner. 

certain  of  reaching  the  right  places. 
Among  the  gymno- 
sperms  (pines,  hem- 
locks, etc.)  this  is  the 
exclusive  method  of 
pollination,  and  when  a 
pine  forest  is  shedding 
pollen  the  air  is  full  of 
the  spores,  which  may 
be  carried  to  a  great 
distance  before  being 
deposited.        Occasional 


Ptelea.—Xfler 


116 


PLANT   STUDIES 


Fig.  118.  Winged  fruit  of 
Ailanthus. —Aiter  Kee- 
ner. 


reports  of  ''showers  of  sulphur ^^  have 
arisen  from  an  especially  heavy  fall  of 
pollen  that  has  been  carried  far  from 
some  gymnosperm  forest.  In  the  case 
of  23ines  and  their  near  relatives,  the 
pollen  spores  are  assisted  in  their  dis- 
persal through  the  air  by  developing  a 
pair  of  broad  wings  from  the  outer 
coat  of  the  spore  (see  Fig.  110).  This 
same  method  of  pollination — that  is, 
carrying  the  pollen  spores  by  currents 
of  air — is  also  used  by  many  mono- 
cotyledons, such  as  grasses  ;  and  by 
many  dicotyledons,  such  as  our  most 


common  forest  trees 
(oak,  hickory,  chest- 
nut, etc.). 

78.  Dispersal  of 
seeds  by  air.— Many 
seeds  are  carried 
about  in  various  ways 
by  currents  of  air 
without  any  special 
adaptation.  Wings 
and  plumes  of  very 
many  and  often  very 
beautiful  patterns 
are  exceedingly  com- 
mon in  connection 
with  seeds  or  seed- 
like fruits  (see  Figs. 
115,  116,  117,  118, 
119).  Wings  are  de- 
veloped by  the  fruit 
of  maples  and  of 
ash,  and  by  the  seeds 


Fig.  119.    Fruit  of  baeswood  {Tilia),  showing  the 
peculiar  wing  formed  by  a  leaf  .—After  Kerner. 


REPRODUCTIVE   ORGAJSS 


117 


Fig.  120.    A  common  tumbleweed  ( C^cto/oma). 


of  pine  and  catalpa.  Plumes  and  tufts  of  hairs  are  devel- 
oped by  the  seed-like  fruits  of  dandelion,  thistle,  and  very 
many  of  their  relatives,  and  by  the  seeds  of  the  milkweed 
(see  Figs.  Ill,  112,  113,  111).  On  plains,  or  level  stretches, 
where  winds  are 
strong,  a  curious 
habit  of  seed  dis- 
persal has  been  de- 
veloped by  certain 
plantsknown  as 
''  tumbleweeds  "  or 
^^field  rollers. '^ 
These  jilants  are 
profusely  branching 
annuals  with  a  small 

.  Fig.  V2l.    The  3-valved  fruit  of  violet  discharging 

root     system     in    a  its  seeds—After  Beal. 


118 


PLANT  STUDIES 


Fig.  122.  A  fruit  of  witch 
hazel  discharging  its 
seeds.— After  Beal. 


light  or  sandy  soil  (see  Fig.  120). 
When  the  work  of  the  season  is  over, 
and  the  absorbing  rootlets  have 
shriveled,  the  plant  is  easily  broken 
from  its  roots  by  a  gust  of  wind, 
and  is  trundled  along  the  surface  like 
a  light  wicker  ball,  the  ripe  seed  ves- 
sels dropping  their  seeds  by  the  way. 
In  case  of  an  obstruction,  such  as  a 
fence,  great  masses  of  these  tumble- 
weeds  may  often  be  seen  lodged 
against  the  windward  side. 

79.  Discharge  of  spores. — In  many 
plants  the  distribution  of  spores  and 
seeds  is  not  provided  for  by  any  of 

the  methods  just  mentioned,  but  the  vessels  containing 

them  are  so  constructed   that  they  are  discharged  with 

more  or  less  violence  and  are  some- 
what scattered. 

Many  spore  cases,  especially  those 

of  the  lower  plants,  burst  irregularly, 

and  with  sufficient  violence  to  throw 

out  spores.     In  the  liverworts  pecu- 
liar cells,  called  elaters  or  '^'^  jumpers," 

are  formed  among  the  spores,  and 

when  the  wall  of  the  spore  case  is 

ruptured  the  elaters  are  liberated, 

and  by  their  active  motion  assist  in 

discharging  the  spores. 

In  most  of  the  true  mosses  the 

spore  case  opens  by  pushing  oif  a 

lid   at   the   apex,    which   exposes   a 

delicate  fringe  of  teeth  covering  the 

mouth  of  the  urn-like  case.     These 

teeth  bend  in  and  out  of  the  open 

spore  case  as  they  become  moist  or 


Fig.  123.  A  pod  of  wild  bean 
bursting,  the  two  valves 
violently  twisting  and  dis- 
charging the  seeds.— After 
Beal. 


REPRODUCTIVE    ORGAXS 


119 


dry,  and  are  of  considerable  service 
in  the  discharge  of  spores. 

In  the  common  ferns  a  heavy 
spring-like  ring  of  cells  encircles 
the  delicate-walled  spore  case. 
When  the  wall  becomes  dry  and 
comparatively  brittle  the  spring 
straightens  with  considerable  force, 
the  delicate  wall  is  suddenly  torn, 
and  in  the  recoil  the  spores  are  dis- 
charged (see  Fig.  45). 

Even  in  the  case  of  the  pollen- 
spores  of  seed-plants,  a  special  layer 
of  the  wall  of  the  pollen-sac  usually 
develops  as  a  spring-like  layer,  which 
assists  in  opening  widely  the  sac 
when  the  wall  be- 
gins to  yield  along 
the  line  of  break- 
ing. 

SO.  Discharge  of 
-While  seeds 


Fig.  li.'5.  A  fruit  of 
beggar  ticks, 
showing  the  two 
barbed  append- 
ageB  which  lay 
hold  of  animals. 
—After  Beal. 

9 


Fig.  124.  Fruits  of  Spanish 
needle,  showing  barbed  ap- 
pendages for  grappling. 
The  figure  to  the  left  is  one 
of  the  fruits  enlarged.— 
After  Kerner. 


are  generally  carried 
away  from  the  parent  plant  by  the  agency 
of  water  currents  or  air  currents,  as  al- 
ready noted,  or  by  animals,  in  some  in- 
stances there  is  a  mechanical  discharge 
provided  for  in  the  structure  of  the  seed- 
case.  In  such  plants  as  the  witch  hazel 
and  violet,  the  walls  of  the  seed-vessel 
press  upon  the  contained  seeds,  so  that 
when  rupture  occurs  the  seeds  are  pinched 
out,  as  a  moist  apple-seed  is  discharged 
by  being  pressed  between  the  thumb  and 
finger  (see  Figs.  121,  122).  In  the  touch- 
me-not  a  strain  is  developed  in  the  wall 
of  the  seed-vessel,  so  that  at  rupture  it 


120 


PLANT   STUDIES 


suddenly  curls  up  and  throws  the  seeds  (see  Fig.  123).  The 
squirting  cucumber  is  so  named  because  it  becomes  very 
much  distended  with  water,  which  is  finally  forcibly  ejected 
along  with  the  mass  of  seed.    An  ^'  artillery  plant "  common 

in  cultivation  discharges  its 
seeds  with  considerable  vio- 
lence ;  while  the  detonations 
resulting  from  the  explosions 
of  the  seed-vessels  of  Hura 
crepitans,  the  ^^  monkey's  din- 
ner bell,"  are  often  remarked 
by  travelers  in  tropical 
forests. 

81.  Dispersal  of  seeds  by  animals. — Only  a  few  illustra- 
tions can  be  given  of  this  very  large  subject.  Water  birds 
are  great  carriers  of  seeds  which  are  contained  in  the  mud 
clinging  to  their  feet  and  legs.  This  mud  from  the  borders 
of  ponds  is  usually  completely  filled  with  seeds  and  spores 
of  various  plants.  One  has  no  conception  of  the  number 
until  they  are  actually  com- 


FiG.  126.  The  fruit  of  carrot,  showing 
the  grappling  appendages.— After 
Beax. 


Fig.  127.    The  fruit  of  cocklebur,  showing 
the  grappling  appendages.— After  Beal. 


puted.  The  following  ex- 
tract from  Darwin's  Origin 
of  Species  illustrates  this 
point  : 

"I  took,  in  February,  three 
tablespoonfuls  of  mud  from  three 
different  points  beneath  water, 
on  the  edge  of  a  little  pond.  This  mud  when  dried  weighed  only  6f 
ounces  ;  I  kept  it  covered  up  in  my  study  for  six  months,  pulling  up 
and  counting  each  plant  as  it  grew  ;  the  plants  were  of  many  kinds, 
and  were  altogether  537  in  number  ;  and  yet  the  viscid  mud  was  all 
contained  in  a  breakfast  cup  !  " 

Water  birds  are  generally  high  and  strong  fliers,  and  the 
seeds  and  spores  may  thus  be  transported  to  the  margins  of 
distant  ponds  or  lakes,  and  so  very  widely  dispersed. 

In  many  cases  seeds  or  fruits  develop  grappling  append- 


REPRODUCTIVE    ORGANS 


121 


ages  of  various  kinds,  which  hiy  hold  of  animals  brushing 
past,  and  so  the  seeds  are  dispersed.  Common  illustrations 
are  Spanish  needles,  beggar  ticks,  stick  seeds,  burdock,  etc. 
Study  Figs.  124,  125,  126,  127,  128,  129,  130. 


Fi6.  128. 


Fruits  with  grappling  appendages.    That  to  the  left  ie  agrimony  ;  that  to 
the  right  is  Galium.— MXev  Kerner. 


In  still  other  cases  the  fruit  becomes  pulpy,  and  attrac- 
tive as  food  to  certain  birds  or  mammals.  Many  of  the 
seeds  (such  as  those  of  grapes)  may  be  able  to  resist  the 
attacks  of  the  digestive  fluids  and  escape  from  the  alimen- 
tary tract  in  a  condition  to  germinate.  As  if  to  attract  the 
attention  of  fruit-eating  animals,  fleshy  fruits  usually 
become  brightly  col- 
ored when  ripe,  so  that 
they  are  plainly  seen 
in  contrast  with  the 
foliage. 

82.  Dispersal  of  pol- 
len spores  by  insects. — 
The  transfer  of  pollen, 

the     name     applied     to      ^'^'^-  ^2^-      ^'""^^^   ^^'^^    grappling   appendages. 
.     .  <•  T  The  figure  to  the  left  is  cocklebur  ;  that  to  the 

certain  spores  of  seed-        right  is  burdock.-After  kerner. 


122 


PLANT  STUDIES 


plants,  is  known  as  pollination,  and 
the  two  chief  agents  of  this  transfer 
are  currents  of  air  and  insects.  In 
§77  the  transfer  by  currents  of  air 
was  noted,  such  plants  being  known 
as  anemopMlous  plants.  Such  plants 
seldom  produce  what  are  generally 
recognized  as  true  flowers.  All  those 
seed-plants  which  produce  more  or 
less  showy  flowers,  however,  are  in 
some  way  related  to  the  visits  of 
insects  to  bring  about  pollination, 
and  are  known  as  e7itomophilous 
plants.  This  relation  between  in- 
sects and  flowers  is  so  important  and  so  extensive  that  .it 
will  be  treated  in  a  separate  chapter. 


Fig.  130.  A  head  of  fruits  of 
burdock,  showing  the 
grappling  appendages.— 
After  Beal. 


CHAPTER  VII 

FLOWERS  AND  INSECTS 

83.  Insects  as  agents  of  pollination. — The  use  of  insects 
as  agents  of  pollen  transfer  is  very  extensive,  and  is  the  pre- 
vailing method  of  pollination  among  monocotyledons  and 
dicotyledons.  All  ordinary  flowers,  as  usually  recognized, 
are  related  in  some  way  to  pollination  by  insects,  but  it 
must  not  be  supposed  that  they  are  always  successful  in 
securing  it.  This  mutually  helpful  relation  between  flow- 
ers and  insects  is  a  very  wonderful  one,  and  in  some  cases 
it  has  become  so  intimate  that  they  cannot  exist  without 
each  other.  Flowers  have  been  modified  in  every  way  to  be 
adapted  to  insect  visits,  and  insects  have  been  variously 
adapted  to  flowers.  , 

84.  Self-pollination  and  cross-pollination. — The  advantage 
of  this  relation  to  the  flower  is  to  secure  pollination.  The 
pollen  may  be  transferred  to  the  carpel  of  its  own  flower, 
or  to  the  carpel  of  some  other  flower.  The  former  is  known 
as  self -'pollination,  the  latter  as  cross-pollination.  In  the 
case  of  cross-pollination  the  two  flowers  concerned  may  be 
upon  the  same  plant,  or  upon  different  plants,  Avhich  may 
be  quite  distant  from  one  another.  It  would  seem  that 
cross-pollination  is  the  preferred  method,  as  flowers  are  so 
commonly  arranged  to  secure  it. 

85.  Advantage  to  insects. — The  advantage  of  this  relation 
to  the  insect  is  to  secure  food.  This  the  flower  provides 
either  in  the  form  of  nectar  ov  pollen  ;  and  insects  visiting 
flowers  may  be  divided  roughly  into  the  two  groups  of 
nectar-feeding  insects,  represented  by  butterflies  and  moths, 

123 


124  PLANT   STUDIES 

and  pollen-feeding  insects,  represented  by  the  numerous 
bees  and  wasps.  AVhen  pollen  is  provided  as  food,  the 
amount  of  it  is  far  in  excess  of  the  needs  of  pollination. 
The  presence  of  these  supplies  of  food  is  made  known  to 
the  insect  by  the  disjDlay  of  color  in  connection  with  the 
flowers,  by  odor,  or  by  form.  It  should  be  said  that  the 
attraction  of  insects  by  color  has  been  doubted  recently,  as 
certain  experiments  have  suggested  that  some  of  the  com- 
mon flower-visiting  insects  are  color-blind,  but  remarkably 
keen-scented.  However  this  may  be  for  some  insects,  it 
seems  to  be  sufficiently  established  that  many  insects  rec- 
ognize their  feeding  ground  by  the  display  of  color. 

86.  Suitable  and  unsuitable  insects. — It  is  evident  that 
all  insects  desiring  nectar  or  pollen  for  food  are  not  suit- 
able for  the  work  of  pollination.  For  instance,  the  ordi- 
nary ants  are  fond  of  such  food,  but  as  they  walk  from  plant 
to  plant  the  pollen  dusted  upon  them  is  in  great  danger  of 
being  brushed  off  and  lost.  The  most  favorable  insect  is 
the  flying  one,  that  can  pass  from  flower  to  flower  through 
the  air.  It  will  be  seen,  therefore,  that  the  flower  must  not 
only  secure  the  visits  of  suitable  insects,  but  must  guard 
against  the  depredations  of  unsuitable  ones. 

87.  Danger  of  self-pollination. — There  is  still  another 
problem  which  insect-pollinating  flowers  must  solve.  If 
cross-pollination  is  more  advantageous  to  the  plant  than 
self-pollination,  the  latter  should  be  prevented  so  far  as 
possible.  As  the  stamens  and  carpels  are  usually  close  to- 
gether in  the  same  flower,  the  danger  of  self-joollination  is 
constantly  present  in  many  flowers.  In  those  plants  which 
have  stamen-producing  flowers  upon  one  plant  and  carpel- 
producing  flowers  upon  another,  there  is  no  such  danger. 

88.  Problems  of  pollination. — In  most  insect-pollinating 
flowers,  therefore,  there  are  three  problems  :  (1)  to  prevent 
self-pollination,  (2)  to  secure  the  visits  of  suitable  insects, 
and  (3)  to  ward  off  the  visits  of  unsuitable  insects.  It 
must  not  be  supposed  that  flowers  are  uniformly  successful 


FLOWERS  AND   INSECTS 


125 


in  solving  these  problems.     They  often  fail,  but  succeed 
often  enough  to  make  the  effort  worth  while. 

89.  Preventing  self-pollination.— It  is  evident  that  this 
danger  arises  only  in  those  flowers  in  which  the  stamens 
and  carpels  are  associ- 
ated, but  their  separa-  ^  '  ^  2 
tion  in  different  flowers 
may  be  considered  as 
one  method  of  prevent- 
ing self-pollination.  In 
order  to  understand  the 
various  arrangements  to 
be  considered,  it  is  necr 
essary  to  explain  that 
the  carpel  does  not  re- 
ceive the  pollen  indif- 
ferently over  its  whole 


There  is  one 
region    organ - 


surface, 
definite 

ized,  known  as  the 
stigma,  upon  which  the 
pollen  must  be  deposited 
if  it  is  to  do  its  work. 
Usually  this  is  at  the 
most  projecting  point 
of  the  carpel,  very  often 
at  the  end  of  a  stalk- 
like prolongation  from 
the  ovary  (the  bulbous 
part  of  the  carpel), 
known  as  the  style; 
sometimes  it  may  run  down  one  side  of  the  style.  AVhen 
the  stigma  is  ready  to  receive  pollen  it  has  upon  it  a 
sweetish,  sticky  fluid,  which  holds  and  feeds  the  pollen. 
In  this  condition  the  stigma  is  said  to  be  mature :  and  the 
pollen  is  mature  when  it  is  being  shed,  that  is,  ready  to  fall 


Fig.  131.  Parts  of  the  flower  of  rose  acacia 
{Robiniahispida).  In  1  the  keel  is  shown  pro- 
jecting from  the  hairy  calyx,  the  other  more 
showy  parts  of  the  corolla  having  been  re- 
moved. Within  the  keel  are  the  stamens 
and  the  carpel,  as  seen  in  3.  The  keel  forms 
the  natural  landing  place  of  a  visiting  bee, 
whose  weight  depresses  the  keel  and  causes 
the  tip  of  the  style  to  protrude,  as  shown  in 
2.  This  style  tip  bears  pollen  upon  it, 
caught  among  the  hairs,  seen  in  3,  and  as  it 
strikes  the  body  of  the  bee  some  pollen  is 
brushed  off.  If  the  bee  has  previously  visited 
another  flower  and  receivetl  some  pollen,  it 
will  be  seen  that  the  stigma,  at  the  very  tip 
of  the  style,  striking  the  body  first,  will  very 
probably  receive  some  of  it.  The  nectar  pit 
is  shown  in  3,  at  the  base  of  the  uppermost 
stamen.— After  Gray. 


126 


PLANT   STUDIES 


out  of  the  pollen-sacs  or  to  be  removed  from  them.  The 
devices  used  by  flowers  containing  both  stamens  and  carpels 
to  prevent  self-pollination  are  very  numerous,  but  most 
of  them  may  be  included  under  the  three  following  heads  : 
(1)  Position. — In  these  cases  the 
pollen  and  stigma  are  ready  at  the  same 
time,  but  their  position  in  reference  to 
each  other,  or  in  reference  to  some  con- 
formation of  the  flower,  makes  it  un- 
likely that  the  pollen  will  fall  upon  the 
stigma.  The  stigma  may  be  placed 
above  or  beyond  the  pollen  sacs,  or  the 
two  may  be  separated  by  some  mechan- 
ical obstruction,  resulting  in  much  of 
the  irregularity  of  flowers. 

In  the  flowers  of  the  rose  acacia  and 
its  relatives,  the  several  stamens  and 
the  single  carpel  are  in  a  cluster,  en- 
closed in  the  keel  of  the  flower.  The 
stigma  is  at  the  summit  of  the  style, 
and  projects  somewhat  beyond  the 
pollen-sacs  shedding  pollen.  Also  there 
is  often  a  rosette  of  hairs,  or  bristles, 
just  beneath  the  stigma,  which  acts  as 
a  barrier  to  the  pollen  (see  Fig.  131). 

In  the  iris,  or  common  flag,  each 
stamen  is  in  a  sort  of  pocket  between 
the  petal  and  the  petal-like  style,  while 
the  stigmatic  surface  is  on  the  toj)  of  a 
flap,  or  shelf,  which  the  style  sends  out 
as  a  roof  to  the  pocket.  With  such  an 
arrangement,  it  would  seem  impossible 
for  the  pollen  to  reach  the  stigma  un- 
aided (see  Fig.  132). 

In  the  orchids,  remarkable  for  their 
strange  and  beautiful  flowers,  there  are 


Fig.  132.  A  portion  of 
the  flower  of  an  iris, 
or  flag.  The  single 
stamen  shown  is 
standing  between  the 
petal  to  the  right  and 
the  petal-like  style  to 
the  left.  Near  the 
top  of  this  style  the 
stigmatic  shelf  is 
seen  extending  to  the 
right,  which  must 
receive  the  pollen 
upon  its  upper  sur- 
face. The  nectar 
pit  is  at  the  junc- 
tion of  the  petal  and 
stamen.  While  ob- 
taining the  nectar  the 
insect  brushes  the 
pollen-bearing  part 
of  the  stamen,  and 
pollen  is  lodged  upon 
its  body.  In  visiting 
the  next  flower  and 
entering  the  stamen 
chamber  the  stig- 
matic shelf  is  apt  to 
be  brushed.— After 
Gray. 


FLOWERS  AND   INSECTS 


127 


usually  two  pollen-sacs,  and  stretched  between  them  is  the 
stigmatic  surface.  In  this  case,  however,  the  pollen  grains 
are  not  dry  and  powdery,  but  cling  together  in  a  mass,  and 
cannot  escape  from  the  sac  without  being  pulled  out  (see 
Fig.  133).  The  same  sort  of  pollen  is  developed  by  the 
milkweeds. 

(2)  Consecutive  maturity. — In  these  cases  the  pollen  and 


Pig.  laS.  A  flower  of  an  orchid  (Habenaria).  At  1  the  complete  flower  is  shown, 
with  three  sepals  behind,  and  three  petals  in  front,  the  lowest  one  of  which  has 
developed  a  long  strap-shaped  portion,  and  a  still  longer  spur  portion,  theoi>ening 
to  which  is  seen  at  the  base  of  the  strap.  At  the  bottom  of  this  long  spur  is  the 
nectar,  which  is  reached  by  the  long  proboscis  of  a  moth.  The  two  pollen  sacs  of 
the  single  stamen  are  seen  in  the  centre  of  the  flower,  diverging  downwards,  and 
between  them  stretches  the  stigma  surface.  The  relation  between  pollen  sacs  and 
stigma  surface  is  more  clearly  shown  in  2.  Within  each  pollen  sac  is  a  mass  of 
sticky  pollen,  ending  below  in  a  sticky  disk,  which  may  be  seen  in  1  and  2.  When 
the  moth  thrusts  his  proboscis  into  the  necUir  tube,  his  head  is  against  the  stig- 
matic surface  and  also  against  the  disks.  When  he  removes  his  head  the  disks 
stick  fast  and  the  pollen  masses  are  dragged  out.  In  3  a  jmllen  mass  (a)  is 
showTi  sticking  to  each  eye  of  a  moth.  I'pon  visiting  another  flower  these  pollen 
masses  are  thrust  against  the  stigmatic  surface  and  pollination  is  effected.— After 
Gray. 


128 


PLANT  STUDIES 


stigma  of  the  same  flower  are  not  mature  at  the  same  time. 
It  is  evident  that  this  is  a  very  effective  method  of  prevent- 
ing self-pollination.  When  the  pollen  is  being  shed  the 
stigma  is  not  ready  to  receive,  or  when  the  stigma  is  ready 
to  receive  the  pollen  is  not  ready  to  be  shed.  In  some 
cases  the  pollen  is  ready  first,  in  other  cases  the  stigma, 
the  former  condition  being  called  protandry,  the  latter 
protogyny.     This  is  a  very  common  method  of  preventing 

self-pollination,  and  is 
usually  not  associated  with 
irregularity. 

The  ordinary  figwort  may 
be  taken  as  an  example  of 
protogyny.  When  the  flow- 
ers first  open,  the  style,  bear- 
ing the  stigma  at  its  tip,  is 
found  protruding  from  the 
urn -like  flower,  while  the 
four  stamens  are  curved 
down  into  the  tube,  and  are 
not  ready  to  shed  their  pollen. 
At  some  later  time  the  style 
bearing  the  stigma  wilts, 
and  the  stamens  straighten 
up  and  protrude  from  the  tube.  In  this  way,  first  the 
receptive  stigma,  and  afterwards  the  shedding  pollen-sacs, 
occupy  the  same  position. 

Protandry  is  even  more  common,  and  many  illustrations 
can  be  obtained.  For  example,  the  showy  flowers  of  the 
common  fireweed,  or  great  willow  herb,  when  first  opened 
display  their  eight  shedding  stamens  prominently,  the  style 
being  sharply  curved  downward  and  backward,  carrying 
the  four  stigma  lobes  well  out  of  the  way.  Later,  the 
stamens  bend  away,  and  the  style  straightens  up  and  ex- 
poses its  stigma  lobes,  now  receptive  (see  Fig.  134). 

(3)  Difference  in  pollen. — In  these   cases  there  are  at 


Fig.  134.  Flowers  of  fireweed  {Epi- 
lobium)^  showing  protandry.  In  1  the 
stamens  are  thrust  forward,  and  the 
style  is  sharply  turned  downward  and 
backward.  In  2  the  style  is  thrust 
forward,  with  its  stigmatic  branches 
spread.  An  insect  in  passing  from  1 
to  2  will  almost  certainly  transfer  pol- 
len from  the  stamens  of  1  to  the  stig- 
mas of  2.— After  Gray. 


FLOWERS   AND    INSECTS 


129 


t. 


.^ 


# 


least  two  forms  of  flowers,  which  differ  from  one  another 
in  the  relative  lengths  of  their  stamens  and  styles.  In  the 
accomjDanying  illustrations  of  Houstonia  (see  Fig.  135)  it 
is  to  be  noticed  that  in  one  flower  the  stamens  are  short 
and  included  in  the  tube,  and  the  style  is  long  and  pro- 
jecting, with  the  four  stigmas  exposed  well  above  the 
tube.  In  the 
other  flower  the 
relative  lengths 
are  exactly  re- 
versed,  the 
style  being 
short  and  in- 
cluded in  the 
tube,  and  the 
stamens  long 
and  projecting. 
It  appears  that 
the  pollen  from 
the  short  sta- 
mens is  most 
effective  upon 
the  stigmas  of 
the  short  styles, 
and  that  the 
pollen  from  the 
long  stamens  is 
most  effective 
upon  tlie  stig- 
mas 

styles,  or  long  stamens  and  sliort  styles,  are  associated  in 
the  same  flower,  the  pollen  must  be  transferred  to  some 
other  flower  to  flnd  its  appropriate  stigma.  Tliis  means 
that  there  is  a  difference  between  the  pollen  of  the  short 
stamens  and  that  of  the  long  ones. 

In  some  cases  there  are  three  forms  of  flowers,  as  in  one 


Fig.  135.  Flowers  of  Houstoyna,  showing  two  forms  of 
flowers.  In  1  there  are  short  stamens  and  a  long  style  ; 
in  2  long  stamens  and  short  style.  An  insect  visiting  1 
will  receive  a  band  of  pollen  about  the  front  part  of  its 
body  ;  upon  visiting  2  this  band  will  rub  against  the 
stigmas,  and  a  fresh  pollen  band  will  be  received  upon 
the  hinder  part  of  the  body,  which,  upon  visiting  another 
flower  like  No.  1,  will  brush  against  the  stigmas. — 
After  Gray. 


of  the  long  styles  ;  and  as  short  stamens  and  long 


130 


PLANT   STUDIES 


of  the  common  loosestrifes.  Each  flower  has  stamens  of 
two  lengths,  which,  with  the  style,  makes  possible  three 
combinations.  One  flower  has  short  stamens,  middle-length 
stamens,  and  long  style  ;  another  has  short  stamens,  middle- 
length  style,  and  long  stamens  ;  the  third  has  short  style, 
middle-length  stamens,  and  long  stamens.  In  these  cases 
also  the  stigmas  are  intended  to  receive  pollen  from  stamens 


Fig,  136.  Yucca  and  Pronuba.  In  the  lower  figure  to  the  right  an  opened  flower 
shows  the  pendent  ovary  with  the  stigma  region  at  its  apex.  The  upper  figure  to 
the  right  shows  the  position  of  Pronuba  when  collecting  pollen.  The  figure  to  the 
left  represents  a  cluster  of  capsules  of  Yticca,  which  shows  the  perforations  made 
by  the  larvae  of  Pronuba  in  escaping. — After  Riley  and  Trelease. 


of  their  own  length,  and  a  transfer  of  pollen  from  flower  to 
flower  is  necessary, 

90.  Self-pollination. — In  considering  these  three  general 
methods  of  preventing  self-pollination,  it  must  not  be  sup- 
posed that  self-pollination  is  never  provided  for.  It  is  pro- 
vided for  more  extensively  than  was  once  supposed.  It  is 
found  that  many  plants,  such  as  violets,  in  addition  to  the 
usual  showy,  insect-pollinated  flowers,  produce  flowers  that 
are  not  at  all  showy,  in  fact  do  not  open,  and  are  often  not 
prominently  placed.  The  fact  that  these  flowers  are  often 
closed   has    suggested   for   them   the    name   cleistogamous 


FLOWERS  AND   INSECTS  131 

flowers.    In  these  flowers  self-pollination  is  a  necessity,  and 
is  found  to  be  very  effective  in  producing  seed. 

91.  Yucca  and  Pronuba. — There  can  be  no  doubt,  also, 
that  there  is  a  great  deal  of  self-pollination  effected  in 
flowers  adapted  for  pollination  by  insects,  and  that  the  in- 
sects themselves  are  often  responsible  for  it.  But  in  the 
remarkable  case  of  Yucca  and  Pronuha  there  is  a  definite 
arrangement  for  self-pollination  by  means  of  an  insect  (see 
Fig.  13G).  Yucca  is  a  plant  of  the  southwestern  arid  regions 
of  Xorth  America,  and  Pronuba  is  a  moth.  The  plant  and 
the  moth  are  very  dependent  upon  each  other.  The  bell- 
shaped  flowers  of  Yucca  hang  in  great  terminal  clusters,  with 
six  hanging  stamens,  and  a  central  ovary  ribbed  lengthwise, 
and  with  a  funnel-shaped  opening  at  its  apex,  which  is  the 
stigma.  The  numerous  ovules  occur  in  lines  beneath  the 
furrows.  During  the  day  the  small  female  Pronuba  rests 
quietly  within  the  flower,  but  at  dusk  becomes  very  active. 
She  travels  down  the  stamens,  and  resting  on  the  open 
pollen-sac  scoops  out  the  somewhat  sticky  pollen  with  her 
front  legs.  Holding  the  little  mass  of  pollen  she  runs  to 
the  ovary,  stands  astride  one  of  the  furrows,  and  pierc- 
ing through  the  wall  with  her  ovipositor,  deposits  an  egg 
in  an  ovule.  After  depositing  several  eggs  she  runs  to  the 
apex  of  the  ovary  and  begins  to  crowd  the  mass  of  pollen 
she  has  collected  into  the  funnel-like  stigma.  These  actions 
are  repeated  several  times,  until  many  eggs  are  deposited 
and  repeated  pollination  has  been  effected.  As  a  result  of 
all  this  the  flower  is  pollinated,  and  seeds  are  formed  which 
develop  abundant  nourishment  for  the  moth  larvae,  which 
become  mature  and  bore  their  way  out  through  the  wall  of 
the  capsule  (Fig.  130). 

92.  Securing  cross-pollination. — In  very  many  ways  flow- 
ers are  adapted  to  the  visits  of  suitable  insects.  In  ob- 
taining nectar  or  pollen  as  food,  the  visiting  insect  receives 
pollen  on  some  part  of  its  body  which  will  be  likely  to 
€ome  in  contact  with  the  stigma  of  the  next  flower  visited. 


Pio.  137.    A  clump  of  lady-elippers  {Cypripediuin),  ehowing  the  habit  or  the  plant 
and  the  general  structure  of  the  flower. — After  Gibson. 


FLOWERS   AND   INSECTS 


133 


Illustrations  of  this  process  may  be  taken  from  the  flowers 
already  described  in  connection  with  the  fjrevention  of 
self-pollination. 

In  the  flowers  of  the  pea  family,  such  as  the  rose  acacia 
(see  Fig.  131),  it  will  be 
noticed  that  the  stamens 
and  pistil  are  concealed 
within  the  keel,  which 
forms  the  natural  land- 
ing place  for  the  bees 
which  are  used  in  pol- 
lination. This  keel  is 
so  inserted  that  the 
weight  of  the  insect  de- 
presses it,  and  the  tip 
of  the  style  comes  in 
contact  with  its  body. 
Not  only  does  the 
stigma  strike  the  body, 
but  by  the  glancing 
blow  the  surface  of  the 
style  is  rubbed  against 
the  insect,  and  on  this 
style,  below  the  stigma, 
the  pollen  has  been  de- 
posited and  is  rubbed 
off  against  the  insect. 
At  the  next  flower 
visited  the  stigma  is 
likely  to  strike  the  pol- 
len obtained  from  the  previous  flower,  and  the  style  will 
deposit  a  new  supply  of  pollen. 

In  the  flower  of  the  common  flag  (see  Fig.  135)  the  nectar 
is  deposited  in  a  pit  at  the  bottom  of  the  chamber  formed 
by  each  style  and  petal.  In  this  chamber  the  stamen  is 
found,  and  more  or  less  roofing  it  over  is  the  flap,  or  shelf. 


Fig.  138.  Flower  of  Cypinpedium.,  showing  the 
flap  overhanging  the  opening  of  the  pouch, 
into  which  a  bee  is  crowding  its  way.  The 
small  figure  to  the  right  shows  a  side  view  of 
the  flap  ;  that  to  the  left  a  view  beneath  the 
flap,  showing  the  two  dark  anthers,  and  be- 
tween them,  further  down  (.forward),  the 
etigma  surface.— After  Gibson. 


134 


PLANT   STUDIES 


upon  the  upper  surface  of  which  the  stigma  is  developed. 
As  the  insect  crowds  its  way  into  this  narrowing  chamber, 
its  body  is  dusted  by  the  pollen,  and  as  it  visits  the  next 
flower  and  thrusts  aside  the  stigmatic  shelf,  it  is  apt  to 
deposit  upon  it  some  of  the  pollen  previously  received. 

The  story  of  pollination  in  connection  with  the  orchids 
is  still  more  complicated  (see  Fig.  133).  Taking  an  ordi- 
nary orchid  for  illustration,  the  details  are  as  follows.  Each 
of  the  two  pollen  masses  terminates  in  a  sticky  disk  or 
button  ;  between  them  extends  the  concave  stigma  sur- 
face, at  the  bottom  of  which  is  the  opening  into  the  long 
tube-like  spur  in  which  the  nectar  is 
found.  Such  a  flower  is  adapted  to 
the  large  moths,  with  long  probosces 
which  can  reach  the  bottom  of  the 
tube.  As  the  moth  thrusts  its  pro- 
boscis into  the  tube,  its  head  touches 
the  sticky  button  on  each  side,  so  that 
when  it  flies  away  these  buttons  stick 
to  its  head,  sometimes  directly  to  its 
eyes,  and  the  pollen  masses  are  torn 
out.  These  masses  are  then  carried 
to  the  next  flower  and  are  thrust 
against  the  stigma  in  the  attempt  to  get  the  nectar. 

In  the  lady-slipper  (Cypripedium),  another  orchid,  the 
flowers  have  a  conspicuous  pouch  (see  Fig.  137),  in  which 
the  nectar  is  secreted.  A  peculiar  structure,  like  a  flap, 
overhangs  the  opening  of  the  pouch,  beneath  which  are  the 
two  anthers,  and  between  them  the  stigmatic  surface  (see 
Fig.  138).  Into  the  pouch  a  bee  crowds  its  way  and  be- 
comes imprisoned  (see  Fig.  139).  The  nectar  which  the 
bee  obtains  is  in  the  bottom  of  the  pouch  (see  Fig.  140). 
When  escaping,  the  bee  moves  towards  the  opening  over- 
hung by  the  flap  and  rubs  first  against  the  stigmatic  sur- 
face (see  Fig.  141),  and  then  against  the  anthers,  receiving 
pollen  on  its  back  (see  Fig.  142).     A  visit  to  another  flower 


Fig.  139.  A  bee  imprisoned 
in  the  pouch  (partly  cut 
away)  of  CypripediutJi. 
— ^After  Gibson. 


FLOWERS  AND   INSECTS 


135 


Fig.  140. 


A  bee  obtaining  nectar  in  the   pouch  of 
Crjpripedlum.—Mikir  Gibson. 


will  result  in  rubbing  some  of  the  pollen  upon  the  stigma, 
and  in  receiving  more  pollen  for  another  flower. 

In  cases  of  protandry,  as  the  common  fig  wort,  flowers 
in  the   two  condi- 
tions will  be  visited 
by  the  pollinating 
insect,  and  as  the 
shedding    stamens 
and  receptive  stig- 
mas   occupy    the 
same  relative  posi- 
tion,   the    pollen 
from    one   flower 
will  be  carried  to  the  stigma  of  another.     It  is  evident  that 
exactly  the  same  methods  prevail  in  the  case  of  protogyny, 
as  the  fireweed  (see  Fig.  134). 

The  Iloustonia  (see  Fig.  135),  in  which  there  are  sta- 
mens and  styles  of  different  lengths,  is  visited  by  insects 

whose  bodies  fill 
the  tube  and  pro- 
trude above  it.  In 
visiting  flowers  of 
both  kinds,  one  re- 
gion of  the  body 
receives  pollen 
from  the  short  sta- 
mens, and  another 
reerion    from    the 


Fig.  141.  A  bee  escaping  from  the  pouch  of  Cijpri- 
pediimi,  and  coming  in  contact  with  the  stigma. 
Advancing  a  little  further  the  bee  will  come  in  con- 
tact with  the  anthers  and  receive  pollen.— After 
Gibson. 


lontr    stamens.      In 


this  way  the  insect 
will  carry  about  two  bands  of  ])ollen,  which  come  in  con- 
tact witli  the  corresponding  stigmas.  Wlien  there  are  three 
forms  of  flowers,  as  mentioned  in  the  case  of  one  of  tlie 
loosestrifes,  the  insect  receives  tliree  pollen  bands,  one  for 
3ach  of  the  three  sets  of  stigmas. 

93.  Warding  off  unsuitable  insects. — Prominent   among 
10 


ioi) 


PLA^T   ISTUI>iES 


the  unsuitable  insects,  which  Kerner  calls  ''unbidden 
guests/'  are  ants,  and  adaptations  for  reducing  their  visits 
to  a  minimum  may  be  taken  as  illustrations. 

(1)  Hairs. — A  common  device  for  turning  back  ants, 
and  other  creeping  insects,  is  a  barrier  of  hair  on  the  stem, 
or  in  the  flower  cluster,  or  in  the  flower. 

(2)  Glandular  secretions. — In  some  cases  a  sticky 
secretion   is   exuded   from   the   surface   of   plants,    which 

effectively  stops 
the  smaller  creep- 
ing insects.  In 
certain  species  of 
catch-fly  a  sticky 
ring  girdles  each 
joint  of  the  stem. 

(3)  Isolation. — 
The  leaves  of  cer- 
tain plants  form 
water  reservoirs 
about  the  stem. 
To  ascend  such  a 
stem,  therefore,  a 
creeping  insect 
must  cross  a  series 
of  such  reservoirs. 
Teasel  furnishes  a 
common  illustration,  the  opposite  leaves  being  united  at 
the  base  and  forming  a  series  of  cups.  More  extensive 
water  reservoirs  are  found  in  Bilhergla  and  Ravenala 
("  traveler's  tree  "),  whose  flower  clusters  are  protected  by 
reservoirs  formed  by  the  rosettes  of  leaves,  which  creeping 
insects  cannot  cross. 

(4)  Latex. — This  is  a  milky  secretion  found  in  some 
plants,  as  in  milkweeds.  Caoutchouc  is  a  latex  secretion 
of  certain  tropical  trees.  When  latex  is  exposed  to  the 
air   it   stiffens   immediately,   becoming  sticky  and  finally 


Fig.  142.  A  bee  escaping  from  the  pouch  of  Cypri- 
pedium,  and  rubbing  against  an  anther.— After 
Gibson. 


FLOWERS   AND   INSECTS  137 

hard.  In  the  flower  clusters  of  many  latex-secreting 
plants  the  epidermis  of  the  stem  is  very  smooth  and  deli- 
cate, and  easily  pierced  by  the  claws  of  ants  and  other 
creeping  insects  who  seek  to  maintain  footing  on  the 
smooth  surface.  Wherever  the  epidermis  is  pierced  the 
latex  gushes  out,  and  by  its  stiffening  and  hardening  glues 
the  insect  fast. 

(5)  Protective  forms. — In  some  cases  the  structure  of 
the  flower  prevents  the  access  of  small  creeping  insects  to 
the  pollen  or  to  the  nectar.  In  the  common  snapdragon 
the  two  lips  are  firmly  closed  (see  Fig.  7-i),  and  they  can  be 
forced  apart  only  by  some  heavy  insect,  as  the  bumble-bee, 
alighting  upon  the  projecting  lower  lip,  all  lighter  insects 
being  excluded.  In  many  species  of  Pentstemon,  one  of 
the  stamens  does  not  develop  pollen  sacs,  but  lies  like  a  bar 
across  the  mouth  of  the  pit  in  which  the  nectar  is  secreted. 
Through  the  crevices  left  by  this  bar  the  thin  proboscis  of 
a  moth  or  butterfly  can  pass,  but  not  the  whole  body  of  a 
creeping  insect.  Very  numerous  adaptations  of  this  kind 
may  be  observed  in  different  flowers. 

(6)  Protective  closure. —  Certain  flowers  are  closed  at 
certain  hours  of  the  day,  when  there  is  the  chief  danger 
from  creeping  insects.  For  instance,  the  evening  prim- 
roses open  at  dusk,  after  the  deposit  of  dew,  when  ants  are 
not  abroad  ;  and  at  the  same  time  they  secure  the  visits  of 
moths,  which  are  night-fliers. 

Numerous  other  adaptations  to  hinder  tlio  visits  of 
unsuitable  insects  may  be  observed,  but  those  given  will 
serve  as  illustrations.  In  all  cases  it  must  be  understood 
that  these  so-called  "  adaptations  "  have  not  been  produced 
to  ward  off  insects,  but  that  having  appeared  from  one 
cause  or  another  they  have  proved  to  be  useful  in  this 
particular. 


CHAPTER  YIU 

AN  INDIVIDUAL  PLANT  IN  ALL  OF  ITS  RELATIONS 

For  the  purpose  of  summarizing  the  general  life-rela- 
tions detailed  in  the  preceding  chapters,  it  will  be  useful  to 
apply  them  in  the  case  of  a  single  plant.  Taking  a  com- 
mon seed-plant  as  an  illustration,  and  following  its  history 
from  the  germination  of  the  seed,  certain  general  facts 
become  evident  in  its  relations  to  the  external  world. 

94.  Germination  of  the  seed. — The  most  obvious  needs  of 
the  seed  for  germination  are  certain  amounts  of  moisture 
and  heat.  In  order  to  secure  these  to  the  best  advantage, 
the  seed  is  usually  very  definitely  related  to  the  soil,  either 
upon  it  and  covered  by  moisture  and  heat-retaining  debris, 
or  embedded  in  it.  Along  with  the  demand  for  heat  and 
moisture  is  one  for  air  (supplying  oxygen),  which  is  essen- 
tial to  life.  The  relation  which  germinating  seeds  need, 
therefore,  is  one  which  not  only  secures  moisture  and  heat 
advantageously,  but  permits  a  free  circulation  of  air. 

95.  Direction  of  the  root. — The  first  part  of  the  young 
plantlet  to  emerge  from  the  seed  is  the  tip  of  the  axis 
which  is  to  develop  the  root  system.  It  at  once  shows  a 
response  to  the  earth  influence  (geotroinsm)  and  to  the 
moisture  influence  {hydrotropism)^  for  whatever  the  direc- 
tion of  emergence  from  the  seed,  a  curvature  is  developed 
which  directs  the  tip  towards  and  flnally  into  the  soil  (see 
Fig.  143).  "When  the  soil  is  penetrated  the  primary  root 
may  continue  to  grow  vigorously  downward,  showing  a 
strong  geotropic  tendency,  and  forming  what  is  known 
as  the  tap-root,  from  which  lateral  roots  arise,  which  are 

138 


AN    INDIVIDUAL   PLANT   IN   ALL   OF   ITS   RELATIONS      139 


much  more  inliuenced  in  direction  by  other  external 
causes,  especially  the  presence  of  moisture.  As  a  rule, 
the  soil  is  not  perfectly  uniform,  and  contact  with  different 
substances  induces  curvatures,  and  as  a  result  of  these  and 
other  causes,  the  root  system  may  become  very  intricate, 
which   is  extremely  favor-  -p 

able   for   absorbing   and 
gripping. 

9G.  Direction  of  the  stem. 
— As  soon  as  the  stem  tip 
is  extricated  from  the  seed, 
it  shows  a  response  to  the 
light  influence  {heliotrop- 
ism)^  being  guided  in  a 
general  way  towards  the 
light  (see  Fig.  143«)- 
Direction  toAvards  the 
light,  the  source  of  the  in- 
fluence, is  spoken  of  as 
positive  heliotropism,  as 
distinguished  from  direc- 
tion away  from  the  light, 
called  negative  heliotro- 
pism. If  the  main  axis 
continues  to  develop,  it 
continues  to  show  this  posi- 
tive heliotropism  strongly, 
but  the  branches  may  show 
every  variation  from  positive  to  transverse  heliotropism  ; 
that  is,  a  direction  transverse  to  the  direction  of  the  rays 
of  light.  In  some  plants  certain  stems,  as  stolons,  run- 
ners, etc.,  show  strong  transverse  heliotropism,  while  other 
stems,  as  rootstocks,  etc.,  show  a  strong  transverse  geot- 
ropism. 

07.  Direction  of  foliage  leaves. — The  general  direction  of 
foliage  leaves  on  an  erect  stem  is  transversely  heliotropic  ; 


Fig.  143.  Germination  of  the  seed  of 
arbor- vitae  (T/iitja).  B  shows  the 
emergence  of  the  axis  (/■)  which  is  to 
develop  the  root,  and  its  turning  to- 
wards the  soil.  C  shows  a  later  stage, 
in  which  the  root  (/•)  has  been  some- 
what developed,  and  the  stem  of  the 
embryo  {h)  is  developing  a  curve  pre- 
paratory to  pulling  out  the  seed  leaves 
(cotyledons).  E  shows  the  young  plant- 
let  entirely  free  from  the  seed,  with  its 
root  (/•)  extending  into  the  soil,  its  stem 
{h)  erect,  and  its  lirst  leaves  (c)  hori- 
zontally spread.— After  Strasburger. 


140  PLANT   STUDIES 

if  necessary,  the  parts  of  the  leaf  or  the  stem  itself  twisting 
to  allow  the  blade  to  assume  this  position.  The  danger  of 
the  leaves  shading  one  another  is  reduced  to  a  minimum  by 
the  elongation  of  internodes,  the  spiral  arrangement,  short- 
ening and  changing  direction  upwards,  or  lobing. 

This  outlines  the  general  nutritive  relations,  the  roots 


Fig.  143a.  Germination  of  the  garden  bean,  showing  the  arch  of  the  seedling  stem 
above  ground,  its  pull  on  the  seed  to  extricate  the  cotyledons  and  plumule,  and 
the  final  straightening  of  the  stem  and  expansion  of  the  young  leaves.— After 
Atkinson. 

and  leaves  being  favorably  placed  for  absorption,  and  the 
latter  also  favorably  placed  for  photosynthesis.  It  is  im- 
portant to  study  the  behavior  of  various  plants  in  the 
germination  of  the  seed,  for  in  a  comparatively  short  period 
all  of  the  important  external  relations  of  the  vegetative 
organs  are  established.  Seeds  should  be  selected  which 
germinate  rapidly,  and  which  represent  different  great 
groups,  such  as  squash,  bean,  corn,  etc.,  and  these  observa- 
tions should  be  extended  as  far  as  possible  by  including  the 
observation  of  seedlings  in  nature. 


AN   INDIVIDUAL   PLANT   IN   ALL   OF   ITS   KELATIONS     141 

98.  Placing  of  flowers. — The  purposes  of  the  flower  seem 
to  be  served  best  by  exposed  positions,  and  consequently 
flowers  mostly  appear  at  the  extremities  of  stems  and 
branches,  a  position  evidently  favorable  to  pollination  and 
seed  dispersal.  The  flowers  thus  exposed  are  very  com- 
monly massed,  or,  if  not,  the  single  flower  is  apt  to  be  large 
and  conspicuous.  The  various  devices  for  protecting  nec- 
tar and  pollen  against  too  great  moisture,  and  the  more 
delicate  structures  against  chill ;  for  securing  the  visits  of 
suitable  insects,  and  warding  off  unsuitable  insects ;  and 
for  dispersing  the  seeds,  need  not  be  repeated. 

99.  Branch  buds. — If  the  plant  under  examination  be  a 
tree  or  shrub,  branch  buds  will  be  observed  to  be  developed 
during  the  growing  season  (see  Fig.  65).  This  device  for 
protecting  growing  tips  through  a  season  of  dangerous  cold 
is  very  familiar  to  those  living  in  the  temperate  regions. 
The  internodes  do  not  elongate,  hence  the  leaves  overlap ; 
they  develop  little  or  no  chlorophyll,  and  become  scales. 
The  protection  afforded  by  these  overlapping  scales  is  often 
increased  by  the  development  of  hairs,  or  by  the  secretion 
of  mucilage  or  gum. 


CHAPTER  IX 

THE   STRUGGLE   FOR  EXISTENCE 

100.  Definition. — The  phrase  '^struggle  for  existence" 
has  come  to  mean,  so  far  as  plants  are  concerned,  that  it  is 
usually  impossible  for  them  to  secure  ideal  relations,  and 
that  they  must  encounter  unfavorable  conditions.  The 
proper  light  and  heat  relations  may  be  difficult  to  obtain, 
and  also  the  proper  relations  to  food  material.  It  often 
happens,  also,  that  conditions  once  fairly  favorable  may  be- 
come unfavorable.  Also,  multitudes  of  plants  are  trying 
to  take  possession  of  the  same  conditions.  All  this  leads 
to  the  so-called  '^'^  struggle,"  and  vastly  more  plants  fail 
than  succeed.  Before  considering  the  organization  of  plant 
associations,  it  will  be  helpful  to  consider  some  of  the 
possible  changes  in  conditions,  and  the  effect  on  plants. 

101.  Decrease  of  water. — This  is  probably  the  most  com- 
mon factor  to  fluctuate  in  the  environment  of  a  plant. 
Along  the  borders  of  streams  and  ponds,  and  in  swampy 
places,  the  variation  in  the  water  is  very  noticeable,  but  the 
same  thing  is  true  of  soils  in  general.  However,  the  change 
chiefly  referred  to  is  that  which  is  permanent,  and  which 
compels  plants  not  merely  to  tide  over  a  drouth,  but  to 
face  a  permanent  decrease  in  the  water  supply. 

Around  the  margins  of  ponds  are  very  commonly  seen 
fringes  of  such  plants  as  bulrushes,  cat-tail  flags,  reed- 
grasses,  etc.,  standing  in  shoal  water.  As  these  plants 
partially  decay,  their  bodies  and  the  entangled  silt  from 
the  land  presently  accumulate  to  such  an  extent  that  there 
is  no  more  standing  water,  and  the  water  supply  for  the 
142 


THE   STRUGGLE    FOR   EXISTENCE  143 

bulrushes  and  their  associates  has  permanently  decreased 
below  the  favorable  amount.  In  this  way  certain  lake 
margins  gradually  encroach  upon  the  water,  and  in  so 
doing  the  Avater  supply  is  permanently  diminished  for  many 
plants.  By  the  same  process,  smaller  lakelets  are  gradually 
being  converted  into  bogs,  and  the  bogs  in  turn  into  drier 
ground,  and  these  unfavorable  changes  in  water  supply  are 
a  menace  to  many  plants. 

The  operations  of  man,  also,  have  been  very  effective  in 
diminishing  the  water  supply  for  plants.  Drainage,  which 
is  so  extensively  practiced,  while  it  may  make  the  water- 
supply  more  favorable  for  the  jilants  which  man  desires,  cer- 
tainly makes  it  very  unfavorable  for  many  other  plants. 
The  clearing  of  forests  has  a  similar  result.  The  forest 
soil  is  receptive  and  retentive  in  reference  to  water,  and  is 
somewhat  like  a  great  sponge,  steadily  supplying  the  streams 
which  drain  it.  The  removal  of  the  forest  destroys  much 
of  this  power.  The  water  is  not  held  and  gradually  doled 
out,  but  rushes  off  in  a  flood ;  hence,  the  streams  which 
drain  the  cleared  area  are  alternately  flooded  and  dried  up. 
This  results  in  a  much  less  total  supply  of  water  available 
for  the  use  of  plants. 

10*2.  Decrease  of  light. — It  is  very  common  to  observe 
tall,  rank  vegetation  shading  lower  forms,  and  seriously 
interfering  with  the  light  supply.  If  the  rank  vegetation 
is  rather  temporary,  the  low  plants  may  learn  to  precede  or 
follow  it,  and  so  avoid  the  shading  ;  but  if  the  over-shading 
vegetation  is  a  forest  growth,  shading  becomes  permanent. 
In  the  case  of  deciduous  trees,  which  drop  their  leaves  at  the 
close  of  the  growing  season  and  put  out  a  fresh  crop  in  the 
spring,  there  is  an  interval  in  the  early  spring,  before  the 
leaves  are  fully  developed,  during  which  low  plants  may 
secure  a  good  exposure  to  light  (see  Fig.  144).  In  such 
places  one  finds  an  abundance  of  ^''spring  flowers/'  but  later 
in  the  season  the  low  plants  become  very  scarce.  This 
effective  over-shading  is  not   common   to   all   forests,  for 


Fig.  144.  A  common  epring  plant  (dog-tooth  violet)  which  grows  in  deciduous 
forests.  The  large  mottled  leaves  and  the  conspicuous  flowers  are  sent  rapidly- 
above  the  surface  from  the  subterranean  bulb  (see  cut  in  the  left  lower  corner), 
where  are  also  seen  dissected  out  some  petals  and  stamens  and  the  pistil. 


THE  STRUGGLE   FOR  EXISTENCE  145 

there  are  ^Miglit  forests,"  such  as  the  oak  forest,  which 
permit  much  low  vegetation,  as  well  as  the  shade  forests, 
such  as  beech  forests,  which  permit  very  little. 

In  the  forest  regions  of  the  tropics,  however,  the  shad- 
ing is  2)ermanent,  since  there  is  no  annual  fall  of  leaves. 
In  such  conditions  the  climbing  habit  has  been  extensively 
cultivated. 

103.  Change  in  temperature. — In  regions  outside  of  the 
tropics  the  annual  change  of  temperature  is  a  very  im- 
portant factor  in  the  life  of  plants,  and  they  have  provided 
for  it  in  one  way  or  another.  In  tracing  the  history  of 
plants,  however,  back  into  what  are  called  "  geological 
times, '^  we  discover  that  there  have  been  relatively  i)er- 
manent  changes  in  temperature.  Now  and  then  glacial 
conditions  prevailed,  during  which  regions  before  temperate 
or  even  tropical  were  subjected  to  arctic  conditions.  It  is 
very  evident  that  such  permanent  changes  of  temperature 
must  have  had  an  immense  influence  upon  plant  life. 

101.  Change  in  soil  composition. — One  of  the  most  ex- 
tensive agencies  in  changing  the  compositions  of  soils  in 
certain  regions  has  been  the  movement  of  glaciers  of  conti- 
nental extent,  which  have  deposited  soil  material  over  very 
extensive  areas.  Areas  within  reach  of  occasional  floods, 
also,  may  have  the  soil  much  changed  in  character  by  the 
new  deposits.  Shifting  dunes  are  billow-like  masses  of 
sand,  developed  and  kept  in  motion  by  strong  prevailing 
winds,  and  often  encroach  upon  other  areas.  Besides  these 
changes  in  the  character  of  soil  by  natural  agencies,  the 
various  operations  of  man  have  been  influential.  Clearing, 
draining,  fertilizing,  all  change  the  character  of  the  soil, 
both  in  its  chemical  composition  and  its  physical  properties. 

105.  Devastating  animals. — The  ravages  of  animals  form 
an  important  factor  in  the  life  of  nuiny  plants.  For  example, 
grazing  animals  are  wholesale  destroyers  of  vegetation,  and 
may  seriously  affect  the  plant  life  of  an  area.  The  various 
leaf  feeders  among  insects   have   frequently  done  a  vast 


146  PLANT   STUDIES 

amount  of  damage  to  plants.  Many  burrowing  animals 
attack  subterranean  parts  of  plants,  and  interfere  seriously 
with  their  occupation  of  an  area. 

Various  protective  adaptations  against  such  attacks  have 
been  pointed  out,  but  this  subject  probably  has  been  much 
exaggerated.  The  occurrence  of  hairs,  prickles,  thorns, 
and  spiny  growths  upon  many  plants  may  discourage  the 
attacks  of  animals,  but  it  would  be  rash  to  assume  that 
these  protections  have  been  developed  because  of  the  danger 
of  such  attacks.  One  of  the  families  of  plants  most  com- 
pletely protected  in  this  way  is  the  great  cactus  family, 
chiefly  inhabiting  the  arid  regions  of  southwestern  United 
States  and  Mexico.  In  such  a  region  succulent  vegetation 
is  at  a  premium,  and  it  is  doubtless  true  that  the  armor  of 
thorns  and  bristles  reduces  the  amount  of  destruction. 

In  addition  to  armor,  the  acrid  or  bitter  secretions  of 
certain  plants  or  certain  parts  of  plants  would  have  a 
tendency  to  ward  off  the  attacks  of  animals. 

106.  Plant  rivalry. — It  is  evident  that  there  must  be 
rivalry  among  plants  in  occupying  an  area,  and  that  those 
plants  which  can  most  nearly  utilize  identical  conditions 
will  be  the  most  intense  rivals.  For  example,  a  great  many 
young  oaks  may  start  up  over  an  area,  and  it  is  evident 
that  the  individuals  must  come  into  sharp  coniiietition  with 
one  another,  and  that  but  few  of  them  succeed  in  establish- 
ing themselves  permanently.  This  is  rivalry  between  in- 
dividuals of  the  same  kind  ;  but  some  other  kind  of  trees, 
as  the  beech,  may  come  into  competition  with  the  oak,  and 
another  form  of  rivalry  will  appear. 

As  a  consequence  of  plant  rivalry,  the  different  plants 
which  finally  succeed  in  taking  possession  of  an  area  are 
apt  to  be  dissimilar,  and  a  plant  association  is  usually  made 
up  of  plants  which  represent  widely  different  regions  of  the 
plant  kingdom.  It  is  sometimes  said  that  any  well-devel- 
oped plant  association  is  an  epitome  of  the  plant  kingdom. 

A  familiar  illustration  of  plant  rivalry  may  be  observed 


THE   STKUGGLE   FOR   EXISTENCE  147 

in  tlie  case  of  what  are  called  ^'^  weeds/'  Every  one  is  fa- 
miliar with  the  fact  that  if  cultivated  ground  is  neglected 
these  undesirable  plants  will  invade  it  vigorously  and  seri- 
ously affect  the  development  of  plants  under  cultivation. 

107.  Adaptation. — When  the  changes  mentioned  above 
occur  in  the  environment  of  j^lants  to  such  an  extent  as 
to  make  the  conditions  for  living  very  unfavorable,  one 
of  three  things  is  likely  to  occur,  adaptation,  migration, 
or  destruction. 

The  change  in  conditions  may  come  slowly  enough,  and 
certain  plants  may  be  able  to  endure  it  long  enough  to 
adjust  themselves  to  it.  Such  an  adjustment  may  involve 
changes  in  structure,  and  probably  no  plants  are  plastic 
enough  to  adjust  themselves  to  extreme  and  sudden  changes 
which  are  to  be  comparatively  permanent.  There  are 
plants,  such  as  the  common  cress,  which  may  be  called 
amphibious,  which  can  live  in  the  water  or  out  of  it  without 
change  of  structure,  but  this  is  endurance  rather  than 
adaptation.  Many  plants,  however,  can  pass  slowly  into 
different  conditions,  such  as  drier  soil,  denser  shade,  etc., 
and  corresponding  changes  in  their  structure  may  be  noted. 
Very  often,  however,  such  plants  are  given  no  opportunity 
to  adjust  themselves  to  the  new  conditions,  as  the  area  is 
apt  to  be  invaded  by  plants  already  better  adapted.  While 
adaptation  may  be  regarded  as  a  real  result  of  changed  con- 
ditions, it  would  seem  to  be  by  no  means  the  common  one. 

108.  Migration. — This  is  a  very  common  result  of 
changed  conditions.  Plants  migrate  as  truly  as  animals, 
though,  of  course,  their  migration  is  from  generation  to 
generation.  It  is  evident,  however,  that  migration  cannot 
be  universal,  for  barriers  of  A-arious  kinds  may  forbid  it. 
In  general,  these  barriers  represent  unfavorable  conditions 
for  living.  If  a  plant  area  with  good  soil  is  surrounded  by 
a  sterile  area,  the  latter  would  form  an  efficient  barrier  to 
migration  from  the  former.  Plants  of  the  lowlands  could 
not  cross  mountains  to  escape  from  unfavorable  conditions. 


148  PLANT  STUDIES 

To  make  migration  possible,  therefore,  it  is  necessary  for 
the  conditions  to  be  favorable  for  the  migrating  plants  in 
some  direction.  In  the  case  of  bulrushes,  cat-tail  flags, 
etc.,  growing  in  the  shoal  water  of  a  lake  margin,  the 
building  up  of  soil  about  them  results  in  unfavorable  con- 
ditions. As  a  consequence,  they  migrate  further  into  the 
lake.  If  the  lake  happens  to  be  a  small  one,  the  filling  up 
process  may  finally  obliterate  it,  and  a  time  will  come  when 
such  forms  as  bulrushes  and  flags  will  find  it  impossible  to 
migrate. 

In  glacial  times  very  many  arctic  plants  migrated  south- 
ward, especially  along  the  mountain  systems,  and  many 
alpine  plants  moved  to  lower  ground.  When  warmer  con- 
ditions returned,  many  plants  that  had  been  driven  south 
returned  towards  the  north,  and  the  arctic  and  alpine  plants 
retreated  to  the  north  and  up  the  mountains.  The  history 
of  plants  is  full  of  migrations,  compelled  by  changed  con- 
ditions and  permitted  in  various  directions.  It  must  be 
remembered,  also,  that  migrations  often  result  in  changes 
of  structure. 

109.  Destruction. — Probably  this  is  by  far  the  most  com- 
mon result  of  greatly  changed  conditions.  Even  if  plants 
adapt  themselves  to  changed  conditions,  or  migrate,  their 
structure  may  be  so  changed  that  they  will  seem  like  quite 
different  plants.  In  this  way  old  forms  gradually  disappear 
and  ncAV  ones  take  their  places. 


CHAPTER  X 

THE   NUTRITION   OF   PLANTS 

110.  Physiology. — In  the  previous  chapters  plants  have 
been  considered  in  reference  to  their  surroundings.  It 
was  observed  that  various  organs  of  nutrition  hold  certain 
life-relations,  but  it  is  essential  to  discover  what  these  rela- 
tions mean  to  the  life  of  the  plant.  The  study  of  plants 
from  the  standpoint  of  their  life-relations  has  been  called 
Ecology  ;  the  study  of  the  life-processes  of  plants  is  called 
Physiology.  These  two  points  of  view  may  be  illustrated 
by  comparing  them  to  two  points  of  view  for  the  study  of 
man.  Man  may  be  studied  in  reference  to  his  relation  to 
his  fellow-men  and  to  the  character  of  the  country  in  which 
he  lives  ;  or  his  bodily  processes  may  be  studied,  such  as 
digestion,  circulation,  respiration,  etc.  The  former  cor- 
responds to  Ecology,  the  latter  is  Physiology. 

All  of  the  ecological  relations  that  have  been  mentioned 
find  their  meaning  in  the  physiology  of  the  plant,  for  life- 
relations  have  in  view  life-processes.  The  subject  of  plant 
physiology  is  a  very  complex  one,  and  it  would  be  impossi- 
ble in  an  elementary  work  to  present  more  than  a  few  very 
general  facts.  Certain  facts  in  reference  to  plant  move- 
ments, an  important  physiological  subject,  have  been  men- 
tioned in  connection  with  life-relations,  but  it  seems  neces- 
sary to  make  some  special  mention  of  nutrition. 

111.  Significance  of  chlorophyll— Probably  the  most  im- 
portant fact  to  observe  in  reference  to  the  nutrition  of 
plants  is  that  some  plants  are  green  or  have  green  parts, 
while  others,  such  as  toadstools,  do  not  show  this  ereen 

149  ^ 


150  PLANT   STUDIES 

color.  It  has  been  stated  that  this  green  color  is  due  to 
the  presence  of  a  coloring  matter  known  as  cliloroyliyll 
(see  §12).  The  two  groups  may  be  spoken  of,  therefore, 
as  (1)  green  "plants  and  (2)  i^lants  ivithout  chlorophyll. 
The  presence  of  chlorophyll  makes  it  possible  for  the  plants 
containing  it  to  manufacture  their  own  food  out  of  such 
materials  as  water,  soil  material,  and  gases.  For  this 
reason,  green  plants  may  be  entirely  independent  of  all 
other  living  things,  so  far  as  their  food  supply  is  concerned. 

Plants  without  chlorophyll,  however,  are  unable  to 
manufacture  food  out  of  such  materials,  and  must  obtain 
it  already  manufactured  in  the  bodies  of  other  plants  or 
animals.  For  this  reason,  they  are  dependent  upon  other 
living  things  for  their  food  supply,  just  as  are  animals.  It 
is  evident  that  plants  without  chlorophyll  may  obtain  this 
food  supply  either  from  the  living  bodies  of  plants  and  ani- 
mals, in  which  case  they  are  called  parasites,  or  they  may 
obtain  it  from  the  substances  derived  from  the  bodies  of 
plants  and  animals,  in  which  case  they  are  called  sapro- 
phytes. For  example,  the  rust  which  attacks  the  wheat, 
and  is  found  upon  the  leaves  and  stems  of  the  living  plant, 
is  a  parasite ;  while  the  mould  which  often  develops  on  stale 
bread  is  a  saprophyte.  Some  plants  without  chlorophyll 
can  live  either  as  parasites  or  saprophytes,  while  others  are 
always  one  or  the  other.  By  far  the  largest  number  of 
parasites  and  saprophytes  belong  to  the  group  of  low  plants 
called  fungi,  and  when  fungi  are  referred  to,  it  must  be 
understood  that  it  means  the  greatest  group  of  plants  with- 
out chlorophyll. 

112.  Photosynthesis. — The  nutritive  processes  in  green 
plants  are  the  same  as  in  other  plants,  and  in  addition  there 
is  in  green  plants  the  peculiar  process  known  as  photosyn- 
thesis (see  §25).  In  plants  with  foliage  leaves,  these  are 
the  chief  organs  for  this  work.  It  must  be  remembered, 
however,  that  leaves  are  not  necessary  for  photosynthesis, 
for  plants  without  leaves,  such  as  algae,  perform  it.      The 


THE   NUTKITION   OF   PLANTS  151 

essential  thing  is  green  tissue  exposed  to  light,  but  in  this 
brief  account  an  ordinary  leafy  plant  growing  in  the  soil 
will  be  considered. 

As  the  leaves  are  the  active  structures  in  the  work  of 
photosynthesis,  the  raw  materials  necessary  must  be  brought 
to  them.  In  a  general  way,  these  materials  are  carbon  di- 
oxide and  water.  The  gas  exists  diffused  through  the 
atmosphere,  and  so  is  in  contact  with  the  leaves.  It  also 
occurs  dissolved  in  the  water  of  the  soil,  but  the  gas  used 
is  absorbed  from  the  air  by  the  leaves.  The  supply  of 
water,  on  the  other  hand,  in  soil-related  plants,  is  obtained 
from  the  soil.  The  root  system  absorbs  this  water,  which 
then  ascends  the  stem  and  is  distributed  to  the  leaves. 

(1)  Ascent  of  ivater. — The  water  does  not  move  up- 
wards through  all  parts  of  the  stem,  but  is  restricted  to  a 
certain  definite  region.  This  region  is  easily  recognized  as 
the  woody  part  of  stems.  Sometimes  separate  strands  of 
wood,  looking  like  fibers,  may  be  seen  running  lengthwise 
through  the  stem  ;  sometimes  the  fibrous  strands  are  packed 
so  close  together  that  they  form  a  compact  woody  mass,  as 
in  shrubs  and  trees.  In  the  case  of  most  trees  new  wood  is 
made  each  year,  through  which  the  water  moves.  Hence 
the  very  common  distinction  is  made  between  sap-icood, 
through  which  the  water  is  moving,  and  heart-wood,  w^hich 
the  water  current  has  abandoned.  Just  how  the  water 
ascends  through  these  woody  fibers,  especially  in  tall  trees, 
is  a  matter  of  much  discussion,  and  cannot  be  regarded  as 
definitely  known.  In  any  event,  it  should  be  remembered 
that  these  woody  fibers  are  not  like  the  open  veins  and 
arteries  of  animal  bodies,  and  no  ''circulation^^  is  possible. 
These  same  woody  strands  are  seen  branching  throughout 
the  leaves,  forming  the  so-called  vein  system,  and  it  is  evi- 
dent, therefore,  that  they  form  a  continuous  route  from 
roots  to  leaves. 

It  is  easy  to  demonstrate  the  ascent  of  water  in  the 
stem,  and  the  path  it  takes,  by  a  simple  experiment.  If 
11 


152  PLANT   STUDIES 

an  active  stem  be  cut  and  plunged  into  water  stained  with 
an  aniline  color  called  eosin,*  the  ascending  water  will  stain 
its  pathway.  After  some  time  sections  through  the  stem 
will  show  that  the  water  has  traveled  upwards  through  it, 
and  the  stain  will  point  out  the  region  of  the  stem  used  in 
the  movement. 

In  general,  therefore,  the  carbon  dioxide  is  absorbed 
directly  from  the  air  by  the  leaves,  and  the  water  is  ab- 
sorbed by  the  root  from  the  soil,  and  moves  upwards  through 
the  stem  into  the  leaves.  An  interesting  fact  about  these 
raw  materials  is  that  they  are  very  common  waste  products. 
They  are  waste  products  because  in  most  life-processes  they 
cannot  be  taken  to  pieces  and  used.     The  fact  that  they 

can  be  used  in  photosynthesis 
shows  that  it  is  a  very  re- 
markable life  process. 

(2)    Cliloroplasts. — Having 

obtained  some  knowledge  of 

the    raw    materials    used    in 

^    ,,     ^  ^^  „     „   *         photosynthesis,    and   their 

Fig.  145.    Some  mesophyll  cells  from      -^  .         . 

the  leaf  of  ii^i^oma,  showing  chloro-      SOUrcCS,      it      is     nCCCSSary     to 

P'^^^^-  consider  the  plant  machinery 

arranged  for  the  work.  In  the  working  leaf  cells  it  is 
discovered  that  the  color  is  due  to  the  presence  of  very 
small  green  bodies,  known  as  chlorophyll  bodies  or  cliloro- 
plasts (see  Fig.  145).  These  consist  of  the  living  substance, 
known  as  protoplasm,  and  the  green  stain  called  chloro- 
phyll ;  therefore,  each  chloroplast  is  a  living  body  ( plastid) 
stained  green.  It  is  in  these  chloroplasts  that  the  work  of 
photosynthesis  is  done.  In  order  that  they  may  work  it 
is  necessary  for  them  to  obtain  a  supply  of  energy  from 
some  outside  source,  and  the  source  used  in  nature  is  sun- 
light. The  green  stain  (chlorophyll)  seems  to  be  used  in 
absorbing   the   necessary  energy  from  sunlight,   and   the 

*  The  commoner  grades  of  red  ink  are  usually  solutions  of  eosin. 


THE   NUTKITION   OF   PLANTS  153 

plastic!  uses  this  energy  in  the  work  of  photosynthesis.  It 
is  evident^  therefore,  that  photosynthesis  goes  on  only  in 
the  sunlight,  and  is  suspended  entirely  at  night.  It  is 
found  that  any  intense  light  can  be  used  as  a  substitute 
for  sunlight,  and  plants  have  been  observed  to  carry  on 
the  work  of  photosynthesis  in  the  presence  of  electric 
light. 

(3)  Result  of  photosynthesis. — The  result  of  this  work 
can  be  stated  only  in  a  very  general  way.  Carbon  dioxide 
is  composed  of  two  elements,  carbon  and  oxygen,  in  the 
proportion  one  part  of  carbon  to  two  parts  of  oxygen. 
Water  is  also  composed  of  two  elements,  hydrogen  and  oxy- 
gen. In  photosynthesis  the  elements  composing  these  sub- 
stances are  separated  from  one  another,  and  recombined  in 
a  new  way.  In  the  process  a  certain  amount  of  oxygen  is 
liberated,  just  as  much  as  was  in  the  carbon  dioxide,  and  a 
new  substance  is  formed,  known  as  a  carbohydrate.  The 
oxygen  set  free  escapes  from  the  plant,  and  may  be  re- 
garded as  waste  product  in  the  process  of  photosynthesis. 
It  will  be  remembered  that  the  external  changes  in  this 
process  are  the  absorption  of  carbon  dioxide  and  the  giving 
off  of  oxygen  (see  §25). 

(4)  Carbohydrates  and  proteids.  —  The  carbohydrate 
formed  is  an  organic  substance ;  that  is,  a  substance  made 
in  nature  only  by  life  processes.  It  is  the  same  kind  of 
substance  as  sugar  or  starch,  and  all  are  known  as  carbohy- 
drates ;  that  is,  substances  composed  of  carbon,  and  of  hy- 
drogen and  oxygen  in  the  same  proportion  as  in  water. 
The  work  of  photosynthesis,  therefore,  is  to  form  carbohy- 
drates. The  carbohydrates,  such  as  sugar  and  starch,  rep- 
resent but  one  type  of  food  material.  Proteids  represent 
another  prominent  type,  substances  which  contain  carbon, 
hydrogen,  and  oxygen,  as  do  carbohydrates,  but  which  also 
contain  other  elements,  notably  nitrogen,  sulphur,  and 
phosphorus.  The  white  of  an  Qgg  may  be  taken  as  an  ex- 
ample of  proteids.     They  seem  to  be  made  from  the  carbo- 


154  PLANT   STUDIES 

hydrates,  the  nitrogen,  sulphur,  and  other  necessary 
additional  elements  being  obtained  from  soil  substances 
dissolved  in  the  water  which  is  absorbed  and  conveyed 
to  the  leaves. 

113.  Transpiration. — The  water  which  is  absorbed  by  the 
roots  and  passes  to  the  leaves  is  much  more  abundant  than 
is  needed  in  the  process  of  photosynthesis.  It  should  be  re- 
membered that  the  water  is  not  only  used  as  a  raw  material 
for  food  manufacture,  but  also  acts  as  a  solvent  of  the  soil 
materials  that  are  passing  into  the  plant.  The  water  in 
excess  of  the  small  amount  used  in  food  manufacture  is 
given  off  from  the  plant  in  the  form  of  water  vapor,  the 
process  being  already  referred  to  as  transiyiration  (see  §26). 

111.  Digestion. — Carbohydrates  and  proteids  may  be  re- 
garded as  prominent  types  of  plant  food  which  green 
plants  are  able  to  manufacture.  These  foods  are  trans- 
ported through  the  plant  to  regions  where  work  is  going  on, 
and  if  there  is  a  greater  supply  of  food  than  is  needed  for 
the  working  regions,  the  excess  is  stored  up  in  some  part 
of  the  plant.  As  a  rule,  green  plants  are  able  to  manufac- 
ture much  more  food  than  they  use,  and  it  is  upon  this  ex- 
cess that  other  plants  and  animals  live.  In  the  transfer  of 
foods  through  the  plant  certain  changes  are  often  neces- 
sary. For  example,  starch  is  insoluble,  and  hence  cannot 
be  carried  about  in  solution.  It  is  necessary  to  transform 
it  into  sugar,  which  is  soluble.  These  changes,  made  to 
facilitate  the  transfer  of  foods,  represent  digestion. 

115.  Assimilation. — When  food  in  some  form  has  reached 
a  working  region,  it  is  organized  into  the  living  substance 
of  the  plant,  known  as  protoplasm,  and  the  protoplasm 
builds  the  plant  structure.  This  process  of  organizing  the 
food  into  the  living  substance  is  known  as  assimilation. 

IIG.  Respiration. — The  formation  of  foods,  their  diges- 
tion and  assimilation  are  all  preparatory  to  the  process  of 
respiration,  which  may  be  called  the  use  of  assimilated 
food.     The  whole  working   power   of  the   plant   depends 


THE   NUTRITION   OF   PLANTS 


155 


upon  respiration,  which  means  the  absorption  of  oxygen  by 
the  protoplasm,  the  breaking  down  of  protoplasm,  and  the 
giving  off  of  carbon  dioxide  and  water  as  wastes.     The  im- 


FiG.  146.  The  common  Northern  pitcher  plant.  The  hollow  leaves,  each  with  a  hood 
and  a  wing,  form  a  rosette,  from  the  center  of  which  arise  the  flower  stalks.— 
After  Kerner. 

portance  of  this  process  may  be  realized  when  it  is  remem- 
bered that  there  is  the  same  need  in  our  own  living,  as  it 
is  essential  for  us  also  to  ''  breathe  in ''  oxygen,  and  as  a 
result  we  '^  breathe  ouf  carbon  dioxide  and  water.  This 
breaking  down  or  ''oxidizing''  of  protoplasm  releases  the 


156 


PLANT   STUDIES 


power  by  which  the  work  of  the  plant  is  carried  on  (see 

§27). 

117.  Summary  of  life-processes. — To  summarize  the  nu- 
tritive life-processes  in  green  j^lants,   therefore,  plwtosyn- 

thesis  manufactures  carbohydrates, 
the  materials  used  being  carbon 
dioxide  and  water,  the  work  being 
done  by  the  chloroplast  with  the 
aid  of  light ;  the  manufacture  of 
proteids  uses  these  carbohydrates, 
and  also  substances  containing 
nitrogen,  sulphur,  etc.;  digestion 
puts  the  insoluble  carbohydrates 
and  the  proteids  into  a  soluble 
form  for  transfer  through  the 
plant;  assimilation  converts  this 
food  material  into  the  living  sub- 
stance of  the  plant,  protoplasm  ; 
respiration  is  the  oxidizing  of  the 
protoplasm  which  enables  the 
plant  to  work,  oxygen  being  ab- 
sorbed, and  carbon  dioxide  and 
water  vapor  being  given  off  in 
the   process. 

118.  Plants  without  chlorophyll. 
— Eemembering  the  life-processes 
described  under  green  plants,  it  is 
evident  that  plants  without  chlo- 

Fia    147.     The  Smithern  pitcher    ..^    j     ^     ^^^^^^^^     ^^    the    WOrk     of 
plant,  showing  the  funnelform         ^    -^ 
and    winged  pitcher,   and    the    photosyuthcsis.       This  mCaUS  that 

overarching  hood  with  trausiu-  |.|-^g„  caunot   manufacture   carbo- 

cent  spots. — After  Keener,  "^ 

hydrates,  and  that  they  must  de- 
pend upon  other  plants  or  animals  for  this  important  food. 
Mushrooms,  puff-balls,  moulds,  mildews,  rusts,  dodder, 
corpse  plants,  beech  drops,  etc.,  may  be  taken  as  illustra- 
tions of  such  plants. 


THE   IsUTEITlOI^    OF  PLANTS  157 

119.  Saprophytes. — In  the  case  of  saprophytes  dead  bodies 
or  body  products  are  attacked,  and  sooner  or  later  all  or- 
ganic matter  is  attacked  and  decomposed  by  them.  The  de- 
composition is  a  result  of  the  nutritive  processes  of  plants 
without  chlorophyll,  and  were  it  not  for  them  "  the  whole  sur- 
face of  the  earth  would  be  covered  with  a  thick  deposit  of 
the  animal  and  plant  remains  of  the  past  thousands  of  years." 

The  green  plants,  therefore,  are  the  manufacturers  of  or- 
ganic material,  producing  far  more  than  they  can  use,  while 
the  plants  without  chlorophyll  are  the  destroyers  of  organic 
material.  The  chief  destroyers  are  the  Bacteria  and  ordi- 
nary Fungi,  but  some  of  the  higher  plants  have  also  adopt- 
ed this  method  of  obtaining  food.  Many  ordinary  green 
plants  have  the  saprophytic  habit  of  absorbing  organic  ma- 
terial from  rich  humus  soil ;  and  such  plants  as  the  broom 
rapes  are  parasitic,  attaching  their  subterranean  parts  to 
those  of  other  plants,  becoming  "  root  parasites." 

120.  Parasites. — Certain  plants  without  chlorophyll  are 
not  content  to  obtain  organic  material  from  dead  bodies, 
but  attack  living  ones.  As  in  the  case  of  saprophytes,  the 
vast  majority  of  plants  which  have  formed  this  habit  are 
Bacteria  and  ordinary  Fungi.  Parasites  are  not  only  modi- 
fied in  structure  in  consequence  of  the  absence  of  chloro- 
phyll, but  they  have  developed  means  of  penetrating  their 
hosts.  Many  of  them  have  also  cultivated  a  very  selective 
habit,  restricting  themselves  to  certain  plants  or  animals, 
or  even  to  certain  organs. 

The  parasitic  habit  has  also  been  developed  by  some  of 
the  higher  plants,  sometimes  completely,  sometimes  par- 
tially. Dodder,  for  example,  is  completely  parasitic  at 
maturity  (Fig.  148),  while  mistletoe  is  only  partially  so, 
doing  chlorophyll  work  and  also  absorbing  from  the  tree 
into  which  it  has  sent  its  haustoria. 

That  saprophytism  and  parasitism  are  both  habits  grad- 
ually acquired  is  inferred  from  the  number  of  green  plants 
which  have  developed  them  more  or  less,  as  a  supplement  to 


158 


PLANT   STUDIES 


the  food  which  they  manufacture.  The  less  chlorophyll  is 
used  the  less  is  it  developed,  and  a  green  plant  which  is 
obtaining  the  larger  amount  of  its  food  in  a  saprophytic 

or  parasitic  way  is 
on  the  way  to  losing 
all  of  its  chlorophyll 
and  becoming  a  com- 
plete saprophyte  or 
parasite. 

Certain  of  the  low- 
er Algae  are  in  the 
habit  of  living  in  the 
body  cavities  of  high- 
er plants,  finding  in 
such  situations  the 
moisture  and  protec- 
tion which  they  need. 
They  may  thus  have 
brought  within  their 
reach  some  of  the 
organic  products  of 
the  higher  plant.  If 
they  can  use  some  of 
these,  as  is  very  like- 
ly, a  partially  para- 
sitic habit  is  begun, 
which  may  lead  to 
loss  of  chlorophyll 
and  complete  para- 
sitism. 

121.  Symbionts. — 
Symdiosis  means 
"living  together," 
and  two  organisms  thus  related  are  called  symUonts.  In 
its  broadest  sense  symbiosis  includes  any  sort  of  depend- 
ence between  living  organisms,  from  the  viae  and  the  tree 


Fig.  148.  A  dodder  plant  parasitic  on  a  willow  twig. 
The  leafless  dodder  twines  about  the  willow,  and 
sends  out  sucking  processes  which  penetrate  and 
absorb.— After  Strasburger. 


THE  NUTRITION  OF  PLANTS  159 

upon  which  it  climbs,  to  the  alga  and  fungus  so  intimately 
associated  in  a  Lichen  as  to  seem  a  single  plant.  In  a 
narrower  sense  it  includes  only  cases  in  which  there  is  an 
intimate  organic  relation  between  the  symbionts.  This 
would  include  parasitism,  the  parasite  and  host  being  the 
symbionts,  and  the  organic  relation  certainly  being  inti- 
mate. In  a  still  narrower  sense  symbiosis  includes  only 
those  cases  in  which  the  symbionts  are  mutually  helpful. 
This  fact,  however,  is  very  difficult  to  determine,  and 
opinions  often  vary  widely  as  to  the  mutual  advantage  in 
certain  cases.  However  large  a  set  of  phenomena  may  be 
included  under  the  term  symbiosis,  we  use  it  here  in  this 
narrowest  sense,  w^hich  is  often  distinguished  as  iiiutualism. 

(1)  Lichens. — A  Lichen  is  a  complex  made  up  of  a  fun- 
gus and  an  alga  living  together.  It  is  certain  that  the 
fungus  cannot  live  without  the  alga,  but  the  alga  can  live 
without  the  fungus.  Hence  it  seems  plain  that  this  rela- 
tion is  not  one  of  mutual  helpfulness,  but  that  the  fungus 
is  living  upon  the  alga  as  any  other  parasite  lives  upon  its 
host  (see  §194). 

(2)  Mycorliiza. — The  name  means  "root-fungus,"  and 
refers  to  an  association  which  exists  between  certain  Fungi 
of  the  soil  and  roots  of  higher  plants.  It  was  formerly 
thought  that  mycorhiza  occurred  only  in  connection  with 
a  limited  number  of  higher  plants,  such  as  orchids,  heaths, 
oaks,  etc.,  but  more  recent  study  indicates  that  probably 
the  large  majority  of  vascular  plants  (that  is,  plants  with 
true  roots)  possess  it,  the  water  plants  being  excepted 
(Figs.  119,  150).  It  has  been  found  that  the  humus  soil  of 
forests  is  in  large  part  "  a  living  mass  of  innumerable  fila- 
mentous fungi.'''  It  is  clearly  of  advantage  to  roots  to 
relate  themselves  to  this  great  network  of  filaments,  which 
are  already  in  the  best  relations  for  absorption,  and  those 
plants  which  are  unable  to  do  this  are  at  a  disadvantage  in 
the  competition  for  the  nutrient  materials  of  the  forest 
soil.    It  is  doubtful  whether  many  vascular  green  plants 


Fig.  149.  Mycorhiza :  to  the  left  is  the  tip  of  a  rootlet  of  beech  enmeshed  by  the 
fungus;  A,  diagram  of  longitudinal  section  of  an  orchid  root,  showing  the  cells 
of  the  cortex  (p)  filled  with  hyphse;  B,  part  of  longitudinal  section  of  orchid  root 
much  enlarged,  showing  epidermis  (e),  outermost  cells  of  the  cortex  (p)  filled  with 
hyphal  threads,  which  are  sending  branches  into  the  adjacent  cortical  cells  (a,  i). 
—After  Feank. 


Fig.  150.  Mycorhiza  :  A,  rootlets  of  white  poplar  forming  mycorrhiza;  B,  enlarged 
section  of  single  rootlets,  showing  the  hyphae  penetrating  the  cells.— After 
Eerkeb. 


THE   NUTRITION  OF  PLANTS 


161 


can  absorb  enough  for  their  needs  from  the  soil  without  this 
assistance,  and,  if  so,  the  fungus  becomes  of  vital  importance 
in  the  nutrition  of  such  plants.  In  the  case  of  some  of  these 
plants  it  seems  that  the  soil  fungus  is  not  merely  passing 
into  their  bodies  the  soil  water  with  its  dissolved  salts,  but  is 
contributing  to  them  organized  food,  thus  diminishing  the 
amount  of  necessary  food  manufac- 
ture. The  delicate  branching  fila- 
ments (hyphae)  of  the  fungus  wrap 
the  rootlets  with  a  mesh  of  hyphae 
and  penetrate  into  the  cells,  and  it 
is  evident  that  the  fungus  obtains 
food  from  the  rootlet  as  a  parasite. 

(3)  Root-tubercles. — On  the  roots 
of  many  legume  plants,  as  clovers, 
peas,  beans,  etc.,  little  wart -like 
outgrowths  are  frequently  found, 
known  as  "  root-tubercles "  (Fig. 
151).  It  is  found  that  these  tuber- 
cles are  caused  by  certain  Bacteria, 
which  penetrate  the  roots  and  in- 
duce these  excrescent  growths.  The 
tubercles  are  found  to  swarm  with 
Bacteria,  which  are  doubtless  ob- 
taining food  from  the  roots  of  the 
host.  At  the  same  time,  these  Bac- 
teria have  the  peculiar  power  of 
laying  hold  of  the  free  nitrogen  of 
the  air  circulating  in  the  soil,  and 
of  supplying  it  to  the  host  plant 
in  some  usable  form.  Ordinarily 
plants  can  not   use  free   nitrogen, 

although  it  occurs  in  the  air  in  such  abundance,  and  this 
power  of  these  soil  Bacteria  is  peculiarly  interesting. 

This  habit  of  clover  and  its  allies  explains  why  they  are 
useful  in  what  is  called  "  restoring  the  soil."    After  ordi- 


FiG.  151.     Root- tubercles  on 
Vicia  Faba.— After  Noll. 


162 


PLANT  STUDIES 


nary  crops  have  exhausted  the  soil  of  its  nitrogen-contain- 
ing salts,  and  it  has  become  comparatively  sterile,  clover  is 
able  to  grow  by  obtaining  nitrogen  from  the  air  through  the 
root-tubercles.  If  the  crop  of  clover  be  "  plowed  under," 
nitrogen-containing  materials  which  tlie  clover  has  organ- 
ized will  be  contributed  to  the  soil,  which  is  thus  restored 
to  a  condition  which  will  support  the  ordinary  crops  again. 
This  indicates  the  significance  of  a  very  ordinary  "  rotation 
of  crops." 

(4)  Ant-plants^  etc. — In  symbiosis  one  of  the  symbionts 
may  be  an  animal.  Certain  fresh-water  polyps  and  sponges 
become  green  on  account  of  Algae  which  they  harbor  with- 
in their  bodies  (Fig.  152).  Like 
the  Lichen -fungus,  these  ani- 
mals are  benefited  by  the  pres- 
ence of  the  Algae,  which  in  turn 
find  a  congenial  situation  for  liv- 
ing. By  some  this  would  also  be 
regarded  as  a  case  of  helotism, 
the  animal  enslaving  the  alga. 

Very  definite  arrangements 
are  made  by  certain  plants  for 
harboring  ants,  which  in  turn 
guard  them  against  the  attack 
of  leaf-cutting  insects  and  oth- 
er foes.  These  plants  are  called 
Myrmecophytes.,  which  means 
"  ant-plants,"  or  myrmecophilous 
plants^  which  means  "plants  loving  ants."  These  plants 
are  mainly  in  the  tropics,  and  in  stem  cavities,  in  hollow 
thorns,  or  elsewhere,  they  provide  dwelling  places  for  tribes 
of  warlike  ants  (Fig.  153).  In  addition  to  these  dwelling 
places  they  provide  special  kinds  of  food  for  the  ants. 

(5)  Flowers  and  insects. — A  very  interesting  and  impor- 
tant case  of  symbiosis  is  that  existing  between  flowers  and 
insects.     The  flowers  furnish  food  to  the  insects,  and  the 


Fig.  152.  A  fresh-water  polyp  {Hy- 
dra) attached  to  a  twig  and  con- 
taining algae  (C),  which  may  be 
seen  through  the  transparent 
body  wall  (5).— Goldberger. 


THE  NUTRITION   OF  PLANTS 


163 


latter  are  used  by  the  flowers  as  agents  of  pollination.     An 
account  of  this  relationship,  with  illustrations,  was  given  in 


Fig.  153.    An  ant  plant  {Ilydnophytum)  from  South  Java,  in  which  an  excrescent 
growth  provides  a  habitation  for  ants.— After  Schimper. 

Chapter  VII,  but  it  should  be  associated  with  other  illustra- 
tions of  symbiosis. 


164 


PLANT  STUDIES 


This  association  of  insects  and  flowers  is  sometimes  so 
intimate  that  they  have  come  to  depend  absolutely  upon 
one  another.  Especially  among  the  orchids  is  it  true  that 
special  flowers  and  insects  are  adapted  so  exactly  to  one 

another,  that  if  one  dis- 
appears the  other  be- 
comes extinct  also. 

122.  "  Carnivorous '" 
plants. — This  name  has 
been  given  to  plants 
which  have  developed 
the  curious  habit  of 
capturing  insects  and 
using  them  for  food, 
and  perhaps  they  had 
better  be  called  "  insec- 
tivorous plants."  They 
are  green  plants  and, 
therefore,  can  manu- 
M  facture  carbohydrates. 
But  they  live  in  soil 
poor  in  nitrogen  com- 
pounds, and  hence  pro- 
teid  formation  is  inter- 
fered with.  The  bodies 
of  captured  insects  sup- 
plement the  proteid 
supply,  and  the  plants 
have  come  to  depend 
upon  them.  Many,  if 
not  all,  of  these  car- 
nivorous plants  secrete 
a  digestive  substance 
which  acts  upon  the 
bodies  of  the  captured  insects  very  much  as  the  diges- 
tive substances  of  the  alimentary  canal  act  upon  proteids 


Fig.  154.  The  Californian  pitcher  plant  {Dar- 
lingtonia),  showing  twisted  and  winged  pitch- 
er, the  overarching  hood  with  translucent 
spots,  and  the  fish-tail  appendage  to  the  hood 
which  is  attractive  to  flying  insects.— After 
Kerner. 


THE  NUTRITION   OF   PLANTS 


165 


swallowed  by  animals.      Some  common  illustrations  are  as 
follows  : 

(1)  Pitcher  plants. — In  these  plants  the  leaves  form 
tubes,  or  urns,  of  various  forms,  which  contain  water,  and 
to  which  insects  are  attracted  and  drowned  (see  Fig.  146). 
A  pitcher  plant  common  throughout  the  Southern  States 
may  be  taken  as  a  type  (see  Fig.  147).  The  leaves  are 
shaped  like  slender,  hollow  cones,  and  rise  in  a  tuft  from 

the  swampy  ground. 
The  mouth  of  this 
conical  urn  is  over- 
arched and  shaded 
by  a  hood,  in  which 
are  translucent  spots, 
like  small  windows. 
Around  the  mouth 
of  the  urn  are 
glands,  which  se- 
crete a  sweet  liquid 
{nectar),  and  nectar 
drops  form  a  trail 
down  the  outside  of 
the  urn.  Inside,  just 
below  the  rim  of  the 
urn,  is  a  glazed  zone, 
so  smooth  that  insects 
cannot  walk  upon  it. 
Below  the  glazed  zone 
is  another  zone, 
thickly  set  with  stiff, 
downward-pointing  hairs,  and  below  this  is  the  liquid  in 
the  bottom  of  the  urn. 

If  a  fly  is  attracted  by  the  nectar  drops  upon  this  curious 
leaf,  it  naturally  follows  the  trail  up  to  the  rim  of  the  urn, 
where  the  nectar  is  abundant.  If  it  attempts  to  descend 
within  the  urn,  it  slips  on  the  glazed  zone,  and  falls  into 


Fig.   155. 


A  sun-dew,  showing  rosette  habit  of 
the  insect-catching  leaves. 


166 


PLANT   STUDIES 


the  water,  and  if  it  attempts  to  escape  by  crawling  up  the 
sides  of  the  urn,  the  thicket  of  downward-pointing  Lairs 
prevents.  If  it  seeks  to  fly  away  from  the  rim,  it  flies 
towards  the  translucent  spots  in  the  hood,  which  look  like 
the  way  of  escape,  as  the  direction  of  entrance  is  in  the 
shadow  of  the  hood.  Pounding  against  the  hood,  the  fly 
falls  into  the  tube.     This  Southern  pitcher  plant  is  known 


Pi».  156.  Two  leaves  of  a  sun-dew.  The  one  to  the  right  has  its  glandular  hairs 
tully  expanded  ;  the  one  to  the  left  shows  half  of  the  hairs  bending  inward,  in  the 
position  assumed  when  an  insect  has  been  captured. — After  Keener. 

as  a  great  fly-catcher,  and  the  urns  are  often  well  supplied 
with  the  decaying  bodies  of  these  insects. 

A  much  larger  Californian  pitcher  plant  has  still  more 
elaborate  contrivances  for  attracting  insects  (see  Fig.  154). 

(2)  Drosera. — The  droseras  are  commonly  known  as 
"  sun-dews,"  and  grow  in  swampy  regions,  the  leaves  form- 
ing small  rosettes  on  the  ground  (see  Fig.  155).  In  one 
form  the  leaf  blade  is  round,  and  the  margin  is  beset  by 
prominent  bristle-like  hairs,  each  with  a  globular  gland  at 
its  tip  (see  Fig.  156).      Shorter   gland-bearing   hairs   ar6> 


C 


THE   NUTKITION   OF  PLANTS 


167 


scattered  also  over  the  inner  surface  of  the  blade.  These 
glands  excrete  a  clear,  sticky  fluid,  which  hangs  to  them  in 
drops  like  dew-drops.     If  a  small  insect  becomes  entangled 


NV 


Fig.  157.    Plants  of  Dioncea,  showing  the  rosette  habit  of  the  leaves  with  terminal 
traps,  and  the  erect  flowering  stem.— After  Keener. 

in  the  sticky  drop,  the  hair  begins  to  curve  inward,  and 
presently  presses  its  victim  down  upon  the  surface  of  the 
blade.  In  the  case  of  larger  insects,  several  of  the  marginal 
hairs  may  join  together  in  holding  it,  or  the  whole  blade 
may  become  more  or  less  rolled  inward. 
12 


168 


PLANT  STUDIES 


(3)  Dioncea. — This  is  one  of  the  most  famous  and  re- 
markable of  fly-catching  plants  (see  Fig.  157).  It  is  found 
in  sandy  swamps  near  AVilmington,  North  Carolina.  The 
leaf  blade  is  constructed  like  a  steel  trap,  the  two  halves 
snapping  together,  and  the  marginal  bristles  interlocking 
like  the  teeth  of  a  trap  (see  Fig.  158).  A  few  sensitive 
hairs,  like  feelers,  are 
developed  on  the  leaf 
surface,  and  when  one 
of  these  is  touched  by 
a  small  flying  or  hover- 
ing insect,  the  trap 
snaps  shut  and  the  in- 
sect is  caught.  Only 
after  digestion  does  the 
trap  open  again. 

There  are  certain 
green  plants,  not  called 
carnivorous  plants, 
which  show  the  same 
general  habit  of  sup- 
plementing their  food 
supply,  and  so  reduc- 
ing the  necessity  of 
food  manufacture. 
The  mistletoe  is  a 
green  plant,  growing 
upon  certain  trees,  from 
which  it  obtains  some  food,  supplementing  that  which  it 
is  able  to  manufacture. 


Fig.  158.  Three  leaves  of  Dioncea^  showing 
the  details  of  the  trap  in  the  leaves  to  right 
and  left,  and  the  central  trap  in  the  act  of 
capturing  an  insect. 


CHAPTER  XI 

PliANT  ASSOCIATIONS:    ECOLOGICAL  FACTORS 

123.  Definition  of  plant  association. — From  the  previous 
chapters  it  has  been  learned  that  every  complex  plant  is  a 
combination  of  organs,  and  that  each  organ  is  related  in 
some  special  way  to  its  environment.  It  follows,  there- 
fore, that  the  whole  plant,  made  up  of  organs,  holds  a  very 
complex  relation  with  its  environment.  The  stem  demands 
certain  things,  the  root  other  things,  and  the  leaves  still 
others.  To  satisfy  all  of  these  demands,  so  far  as  possible, 
the  whole  plant  is  delicately  adjusted. 

The  earth's  surface  presents  very  diverse  conditions  in 
reference  to  plant  life,  and  as  plants  are  grouped  according 
to  these  conditions,  this  leads  to  definite  associations  of 
plants,  those  adapted  to  the  same  general  conditions  being 
apt  to  live  together.  Such  an  assemblage  of  plants  living 
together  in  similar  conditions  is  21^  plant  association,  the  con- 
ditions forbidding  other  plants.  It  must  not  be  understood 
that  all  plants  affecting  the  same  conditions  will  be  found  liv- 
ing together.  For  example,  a  meadow  of  a  certain  typo  will 
not  contain  all  the  kinds  of  grasses  associated  with  that  type. 
Certain  grasses  will  be  found  in  one  meadow,  and  otiier 
grasses  will  be  found  in  other  meadows  of  the  same  type. 

The  rivalry  of  closely  related  plants  living  in  the  same 
association  is  apt  to  be  intense,  on  account  of  their  similar 
demands,  and  unrelated  plants  are  able  to  live  together  with 
the  least  rivalry.  A  plant-  association,  therefore,  may  con- 
tain a  wide  representation  of  the  plant  kingdom,  from 
plants  of  low  rank  to  those  of  high  rank. 

169 


170  PLANT  STUDIES 

Before  considering  some  of  the  common  associations,  it 
is  necessary  to  note  some  of  the  conditions  which  detei  • 
mine  plant  associations.  Those  things  in  the  environment 
of  the  plant  which  influence  the  organization  of  an  associa- 
tion are  known  as  ecological  factors. 

124.  Water. — Water  is  certainly  one  of  the  most  im- 
portant conditions  in  the  environment  of  a  plant,  and  has 
great  influence  in  determining  the  organization  of  associa- 
tions. If  all  plants  are  considered,  it  will  be  noted  that  the 
amount  of  water  to  which  they  are  exposed  is  exceedingly 
variable.  At  one  extreme  are  those  plants  which  are  com- 
pletely submerged ;  at  the  other  extreme  are  those  plants 
of  arid  regions  which  can  obtain  very  little  water ;  and  be- 
tween these  extremes  there  is  every  gradation  in  the  amount 
of  available  water.  Among  the  most  striking  adaptations 
of  plants  are  those  for  living  in  the  presence  of  a  great 
amount  of  water,  and  those  for  guarding  against  its  lack. 

One  of  the  first  things  to  consider  in  connection  with 
any  plant  association  is  the  amount  of  water  supply.  It  is 
not  merely  a  question  of  its  total  annual  amount,  but  of  its 
distribution  through  the  year.  Is  it  supplied  somewhat 
uniformly,  or  is  there  alternating  flood  and  drouth  ?  The 
nature  of  the  water  supply  is  also  important.  Are  there 
surface  channels  or  subterranean  channels,  or  does  the 
whole  supply  come  in  the  form  of  rain  and  snow  which 
fall  upon  the  area? 

Another  important  fact  to  consider  in  connection  with 
the  water  supply  has  to  do  with  the  structure  of  the  soil. 
There  is  what  may  be  called  a  water  level  in  soils,  and  it  is 
important  to  note  the  depth  of  this  level  beneath  the  sur- 
face. In  some  soils  it  is  very  near  the  surface ;  in  others, 
such  as  sandy  soils,  it  may  be  some  distance  beneath  the 
surface. 

!N'ot  only  do  the  amount  of  water  and  the  depth  of  the 
yrater  level  help  to  determine  plant  associations,  but  also  the 
substances  which  the  water  contains.     Two  areas  may  have 


PLANT  ASSOCIATIONS:  ECOLOGICAL  FACTORS    171 

the  same  amount  of  water  and  the  same  water  level,  but  if 
the  substances  dissolved  in  the  water  differ  in  certain  par- 
ticulars, two  entirely  distinct  associations  may  result. 

125.  Heat. — The  general  temperature  of  an  area  is  im- 
portant to  consider,  but  it  is  evident  that  differences  of 
temperature  are  not  so  local  as  differences  in  the  water  sup- 
ply, and  therefore  this  factor  is  not  so  important  in  the 
organization  of  the  plant  associations  of  any  given  neigh- 
borhood as  is  the  water  factor.  Even  in  the  distribution 
of  plants  over  the  surface  of  the  earth,  however,  the  water 
factor  is  probably  more  important  than  the  heat  factor.  The 
range  of  temperature  which  the  plant  kingdom,  as  a  whole, 
can  endure  during  active  work  may  be  stated  in  a  general 
way  as  from  0°  to  50°  C. ;  that  is,  from  the  freezing  point 
of  water  to  122°  Fahr.  There  are  certain  plants  which  can 
work  at  higher  temperatures,  notably  certain  alg^e  growing 
in  hot  springs,  but  they  may  be  regarded  as  exceptions.  It 
must  be  remembered  that  the  range  of  temperature  given 
is  for  plants  actively  at  work,  and  does  not  include  the  tem- 
perature which  many  plants  are  able  to  endure  in  a  specially 
protected  but  very  inactive  condition.  For  examjole,  many 
plants  of  the  temperate  regions  endure  a  winter  tempera- 
ture which  is  frequently  lower  than  the  freezing  point  of 
water,  but  it  is  a  question  of  endurance  and  not  of  work. 

It  must  not  be  supposed  that  all  plants  can  work  equally 
well  throughout  the  whole  range  of  temperature  given,  for 
they  differ  widely  in  this  regard.  Tropical  plants,  for  in- 
stance, accustomed  to  a  certain  limited  range  of  high  tem- 
perature, cannot  work  continuously  at  the  lower  tempera- 
tures. For  each  kind  of  plant  there  is  what  may  be  called 
a  zero  point,  below  which  it  is  not  in  the  habit  of  working. 

While  it  is  important  to  note  the  general  temperature 
of  an  area  throughout  the  year,  it  is  also  necessary  to  note 
its  distribution.  Two  regions  may  have  presumably  the 
same  amount  of  heat  tlirough  the  year,  but  if  in  the  one  case 
it  is  uniformly  distributed,  and  in  the  other  great  extremes 


172  PLANT   STUDIES 

of  temperature  occur,  the  same  plants  will  not  be  found  in 
both.  It  is,  perhaps,  most  important  to  note  the  tempera- 
ture during  certain  critical  periods  in  the  life  of  plants, 
such  as  the  flowering  period  of  seed-plants. 

Although  the  temperature  problem  may  be  compara- 
tively uniform  over  any  given  area,  the  effect  of  it  may  be 
noted  in  the  succession  of  plants  through  the  growing  sea- 
son. In  our  temperate  regions  the  spring  j^lants  and  summer 
plants  and  autumn  plants  differ  decidedly  from  one  another. 
It  is  evident  that  the  spring  plants  can  endure  greater 
cold  than  the  summer  plants,  and  the  succession  of  flowers 
will  indicate  somewhat  these  relations  of  temperature. 

It  should  be  remarked,  also,  that  not  only  is  the  tem- 
perature of  the  air  to  be  noted,  but  also  that  of  the  soil. 
These  two  temperatures  may  differ  by  several  degrees,  and 
the  soil  temperature  especially  affects  root  activity,  and 
hence  is  a  very  important  factor  to  discover. 

At  this  point  it  is  possible  to  call  attention  to  the  effect 
of  the  combination  of  ecological  factors.  For  instance,  in 
reference  to  the  occurrence  of  plants  in  any  association,  the 
water  factor  and  the  heat  factor  cannot  be  considered  each 
by  itself,  but  must  be  taken  in  combination.  For  example, 
if  in  a  given  area  there  is  a  combination  of  maximum  heat 
and  minimum  water,  the  result  will  be  a  desert,  and  only 
certain  specially  adapted  plants  can  exist.  It  is  evident 
that  the  great  heat  increases  the  transpiration,  and  trans- 
piration when  the  supply  of  water  is  very  meager  is  pecu- 
liarly dangerous.  Plants  which  exist  in  such  conditions, 
therefore,  must  be  specially  adapted  for  controlling  trans- 
piration. On  the  other  hand,  if  in  any  area  the  combina- 
tion is  maximum  heat  and  maximum  water,  the  result  will 
be  the  most  luxuriant  vegetation  on  the  earth,  such  as 
grows  in  the  rainy  tropics.  It  is  evident  that  the  possible 
combinations  of  the  water  and  heat  factors  may  be  very 
numerous,  and  that  it  is  such  combinations  that  chiefly 
affect  plant  associations. 


PLANT   ASSOCIATIONS:    ECOLOGICAL  FACTORS  173 

126.  Soil — The  soil  factor  is  not  merely  important  to 
consider  in  connection  with  those  plants  directly  related 
to  the  soil,  but  is  a  factor  for  all  plants,  as  it  determines 
the  substances  which  the  water  contains.  There  are  two 
things  to  be  considered  in  connection  with  the  soil,  namely, 
its  chemical  composition  and  its  physical  properties.  Per- 
haps the  physical  properties  are  more  important  from  the 
standpoint  of  soil-related  plants  than  the  chemical  com- 
position, although  both  the  chemical  and  physical  nature 
of  the  soil  are  so  bound  up  together  that  they  need  not  be 
considered  separately  here.  The  physical  properties  of  the 
soil,  which  are  important  to  plants,  are  chiefly  those  which 
relate  to  the  water  supply.  It  is  always  important  to  de- 
termine how  receptive  a  soil  is.  Does  it  take  in  w^ater 
easily  or  not  ?  It  is  also  necessary  to  determine  how  re- 
tentive it  is  ;  it  may  receive  water  readily,  but  it  may  not 
retain  it. 

For  convenience  in  ordinary  field  work  with  plants, 
soils  may  be  divided  roughly  into  six  classes  :  (1)  rock, 
w^hich  means  solid  uncrumbled  rock,  upon  which  certain 
plants  are  able  to  grow  ;  (2)  sayid,  which  has  small  water 
capacity,  that  is,  it  may  receive  water  readily  enough,  but 
does  not  retain  it ;  (3)  lime  soil ;  (4)  clay,  which  has  great 
water  capacity  ;  (5)  humus,  which  is  rich  in  the  products 
of  plant  and  animal  decay  ;  (6)  salt  soil,  in  which  the  water 
contains  certain  salts,  and  is  generally  spoken  of  as  alka- 
line. These  divisions  in  a  rough  way  indicate  both  the 
structure  of  the  soil  and  its  chemical  composition.  Not 
only  should  the  kinds  of  soil  on  an  area  be  determined, 
but  their  depth  is  an  important  consideration.  It  is 
very  common  to  find  one  of  these  soils  overlying  another 
one,  and  this  relation  between  the  two  will  have  a  very 
important  effect.  For  instance,  if  a  sand  soil  is  found 
lying  over  a  clay  soil,  the  result  will  be  that  the  sand  soil 
will  retain  far  more  water  than  it  would  alone.  If  a  humus 
soil  in  one  area  overlies  a  sand  soil,  and  in  another  area 


174  PLANT   STUDIES 

overlies  a  clay  soil,  the  humus  will  differ  very  much  in  the 
two  cases  in  reference  to  water. 

The  soil  cover  should  also  be  considered.  The  common 
soil  covers  are  snow,  fallen  leaves,  and  living  plants.  It 
will  be  noticed  that  all  these  covers  tend  to  diminish  the 
loss  of  heat  from  the  soil,  as  well  as  the  access  of  heat  to 
the  soil.  In  other  words,  a  good  soil  cover  will  very  much 
diminish  the  extremes  of  temj^erature.  All  this  tends  to 
increase  the  retention  of  water. 

127.  Light. — It  is  known  that  light  is  essential  for  the 
peculiar  work  of  green  plants.  However,  all  green  plants 
cannot  have  an  equal  amount  of  light,  and  some  have 
learned  to  live  with  a  less  amount  than  others.  While 
no  sharp  line  can  be  drawn  between  green  plants  which 
use  intense  light,  and  those  which  use  less  intense  light, 
we  still  recognize  in  a  general  way  what  are  called  light 
plants  and  shade  plants.  We  know  that  certain  plants 
are  chiefly  found  in  situations  where  they  can  be  exposed 
freely  to  light,  and  that  other  plants,  as  a  rule,  are  found 
in  shady  situations. 

Starting  with  this  idea,  we  find  that  plants  grow  in 
strata.  In  a  forest  association,  for  example,  the  tall  trees 
represent  the  highest  stratum ;  below  this  there  may  be  a 
stratum  of  shrubs,  then  tall  herbs,  then  low  herbs,  then 
forms  like  mosses  and  lichens  growing  close  to  the  ground. 
In  any  plant  association  it  is  important  to  note  the  num- 
ber of  these  strata.  It  may  be  that  the  highest  stratum 
shades  so  densely  that  many  of  the  other  strata  are  not 
represented  at  all.  An  illustration  of  this  can  be  obtained 
from  a  dense  beech  forest. 

128.  Wind. — It  is  generally  known  that  wind  has  a  dry- 
ing effect,  and,  therefore,  it  increases  the  transpiration  of 
plants  and  tends  to  impoverish  them  in  water.  This  fac- 
tor is  especially  conspicuous  in  regions  where  there  are  pre- 
vailing winds,  such  as  near  the  sea-coast,  around  the  great 
lakes,  and  on  the  prairies  and  plains.     In  all  such  regions 


PLANT   ASSOCIATIONS:    ECOLOGICAL  FACTORS  175 

the  plants  have  been  compelled  to  adapt  themselves  to  this 
loss  of  water ;  and  in  some  regions  the  prevailing  winds 
are  so  constant  and  violent  that  the  force  of  the  -wind  itself 
has  influenced  the  appearance  of  the  vegetation,  giving 
what  is  called  a  characteristic  physiognomy  to  the  area. 

These  five  factors  have  been  selected  from  a  much 
larger  number  that  might  be  enumerated,  but  they  may 
be  regarded  as  among  the  most  important  ones.  It  will  be 
noticed  that  these  factors  may  be  combined  in  all  sorts 
of  ways,  so  that  an  almost  endless  series  of  combinations 
seems  to  be  possible.  This  will  give  some  idea  as  to  the 
possible  number  of  plant  associations,  for  they  may  be  as 
numerous  as  are  the  combinations  of  these  factors. 

129.  The  great  groups  of  associations. — It  is  possible  to 
reduce  the  very  numerous  associations  to  three  or  four 
great  groups.  For  convenience,  the  water  factor  is  chiefly 
used  for  this  classification.  It  results  in  a  convenient 
classification,  but  one  that  is  certainly  more  or  less  arti- 
ficial. The  selection  of  any  one  factor  from  among  the 
many  for  the  purpose  of  classification  never  results  in  a 
very  natural  classification  when  the  combination  of  factors 
determines  the  group.  However,  for  general  purposes,  the 
usual  classification  on  the  basis  of  water  supply  w411  be 
used.  On  this  basis  there  are  three  great  groups  of  asso- 
ciations, as  follows : 

(1)  Hydrophytes. — The  name  means  "  water  plants,"  and 
suggests  that  such  associations  are  at  that  extreme  of  the 
water  supply  where  it  is  very  abundant.  Such  plants  may 
grow  in  the  water,  or  in  very  wet  soil,  but  in  any  event 
they  are  exposed  to  a  large  amount  of  water. 

(2)  Xerophytes. — The  name  means  "  drouth  plants,"  and 
suggests  the  other  extreme  of  the  water  supply.  True 
xerophytes  are  exposed  to  dry  soil  and  dry  atmosphere. 

(3)  Mcsophytes. — Between  the  two  extremes  of  the 
water  supply  there  is  a  great  middle  region  of  medium 
water  supply,  and  plants  which  occupy  it  are  known  as 


176  PLANT   STUDIES 

mesophytes,  the  plants  of  medium  conditions.  It  is  evi- 
dent that  mesophytes  gradually  pass  into  hydrophytes  on 
the  one  side,  and  into  xerophytes  on  the  other;  but  it  is 
also  evident  that  mesophyte  associations  have  the  greatest 
range  of  water  supply,  extending  from  a  large  amount  of 
water  to  a  very  small  amount. 

It  should  be  understood  that  these  three  groups  of 
associations,  which  are  distinguished  from  one  another  by 
the  amount  of  the  water  supply,  are  artificial  groups  rather 
than  natural  ones,  for  they  bring  together  unrelated  asso- 
ciations, and  often  separate  those  that  are  closely  related. 
For  example,  a  swampy  meadow  is  put  among  hydrophyte 
associations  by  this  classification ;  and  it  may  shade  into  an 
ordinary  meadow,  which  belongs  among  the  mesophytes. 
Probably  the  largest  fact  which  may  be  used  in  grouping 
plant  associations  is  that  certain  associations  are  so  situ- 
ated that  they  seek  for  the  most  part  to  reduce  transpira- 
tion, and  that  others  are  so  situated  that  they  seek  for  the 
most  part  to  increase  transpiration. 

However,  the  factors  which  determine  associations  are 
so  numerous  that  they  cannot  be  presented  in  an  elementary 
book,  and  the  simpler  artificial  grouping  given  above  will 
serve  to  introduce  the  associations  to  observation. 


CHAPTER  XII 

HYDROPHYTE   ASSOCIATIONS 

130.  General  character. — Hydrophytes  are  related  to 
abundant  water,  either  throughout  their  whole  structure 
or  in  part  of  their  structure.  It  is  a  well-known  fact  that 
hydrophytes  are  among  the  most  cosmopolitan  of  plants, 
and  hydrophyte  associations  in  one  part  of  the  world  look 
very  much  like  hydrophyte  associations  in  any  other 
region.  It  is  probable  that  the  abundant  water  makes  the 
conditions  more  uniform. 

It  is  evident  that  for  those  plants,  or  plant  parts,  which 
are  submerged,  the  water  affects  the  heat  factor  by  dimin- 
ishing the  extremes.  It  also  affects  the  light  factor,  in  so 
far  as  the  light  must  pass  through  the  water  to  reach  the 
chlorophyll-containing  parts,  as  light  is  diminished  in  in- 
tensity by  passing  through  the  water.  Before  considering 
a  few  hydrophyte  associations,  it  is  necessary  to  note  the 
prominent  hydrophyte  adaptations. 

131.  Adaptations. — In  order  that  the  illustration  may  be 
as  simple  as  possible,  a  complex  plant  completely  exposed 
to  water  is  selected,  for  it  is  evident  that  the  relations  of  a 
swamp  plant,  with  its  roots  in  water  and  its  stem  and  leaves 
exposed  to  air,  are  complicated.  A  number  of  adaptations 
may  be  noted  in  connection  with  the  submerged  or  floating 
plant. 

(1)  Thin-imlled  epidermis. — In  the  case  of  the  soil-re- 
lated plants,  the  water  supply  comes  mainly  from  the  soil, 
and  the  root  system  is  constructed  to  absorb  it.  In  the 
case  of  the  water  plant  under  consideration,  however,  the 


178 


PLANT   STUDIES 


whole  plant  body  is  exposed  to  the  water  supply,  and  there- 
fore absorption  may  take  place  through  the  whole  surface 
rather  than  at  any  particular  region  such  as  the  root.  In 
order  that  this  may  be  done,  however,  it  is  necessary  for 
the  epidermis  to  have  thin  walls,  which  is  usually  not  the 
case  in  epidermis  exposed 
to  the  air,  where  a  certain 
amount  of  protection  is 
needed  in  the  way  of 
thickening. 

(2)  Roots  much  reduced 
or  wa7itmg. — It  must  be 
evident  that  if  water  is 
being  absorbed  by  the 
whole  free  surface  of  the 
plant,  there  is  not  so 
much  need  for  a  special 
root  region  for  absorp- 
tion. Therefore,  in  such 
Avater  plants  the  root  sys- 
tem may  be  much  re- 
duced, or  may  even  disap- 
pear entirely.  It  is  often 
retained,  however,  to  act 
as  a  holdfast,  rather  than 
as  an  absorbent  organ,  for 
most  water  j^lants  anchor 
themselves  to  some  sup- 
port. 

(3)  Reduction  of  IV ater -conducting  tissues. — In  the  ordi- 
nary soil-related  plants,  not  only  is  an  absorbing  root  sys- 
tem necessary,  but  also  a  conducting  system,  to  carry  the 
water  absorbed  from  the  roots  to  the  leaves  and  elsewhere. 
It  has  already  been  noted  that  this  conducting  system  takes 
the  form  of  woody  strands.  It  is  evident  that  if  water 
is  being  absorbed  by  the  whole  surface  of  the  plant,  the 


Fig.  159.  Fragment  of  a  common  seaweed 
(Fucus)^  showing  the  body  with  forliing 
branching  and  bladder-like  air  cavities. — 
After  LuERSSEN. 


HYDROPHYTE  ASSOCIATIONS 


179 


work  of  conduction  is  not  so  extensive  or  definite,  and 
therefore  in  such  water  plants  the  woody  bundles  are  not 
so  prominently  developed  as  in  land  plants. 

(•4)  Reduction   of  mechanical  tissues. — In   the   case   of 
ordinary  land  j^lants,  certain  firm  tissues  are  developed  so 


Fio.  IGO.     Gulfweed  {Sargassiim),  showing  the  thallus  differentiated  into  stem-like  and 
leaf-like  portions,  and  also  the  liladder-like  floats.— After  Bennett  and  Mi-rray. 

that  the  plant  may  maintain  its  form.  These  supporting 
tissues  reach  their  culmination  in  such  forms  as  trees, 
where  massive  bodies  are  able  to  stand  upriglit.  It  is  evi- 
dent that  in  the  water  there  is  no  such  need  for  rigid  sup- 
porting tissues,  as  the  buoyant  power  of  water  helps  to 
support  the  plant.     This  fact  may  be  illustrated  by  taking 


180 


PLANT   STUDIES 


out  of  water  submerged  plants  which  seem  to  be  upright, 
with  all  their  parts  properly  spread  out.   When  removed  they 
collapse,  not  being  able  to  support  themselves  in  any  way. 
(5)  Development  of  air  cavities. — The  presence  of  air  in 
the  bodies  of  water  plants  is  necessary  for  two  reasons:  (1), 


^-Z^^% 


,.,^ 


i^ 


Fig.  161.  Bladderwort,  showing  the  numerous  bladders  which  float  the  plant,  the 
finely  divided  water  leaves,  and  the  erect  flowering  stems.  The  bladders  are  also 
effective  "insect  traps,"  Utricularia  being  one  of  the  "carnivorous  plants." 
—After  Keener. 


to  aerate  the  plant ;  (2),  to  increase  its  buoyancy.  In  most 
complex  water  plants  there  must  be  some  arrangement  for 
the  distribution  of  air  containing  oxygen.  This  usually 
takes  the  form  of  air  chambers  and  passageways  in  the 
body  of  the  plant  (see  Figs.  87,  88,  89,  90).  Of  course 
euch  air  chambers  increase  the  buoyancy  of  the  body. 
Sometimes,  however,  a  special  buoyancy  is  provided  for 
by  the  development  of  regular  floats,  which  are  bladder^ 


HYDKOPIIYTE   ASSOCIATIONS 


181 


like  bodies  (see  Figs.  159,  160).  These  floats  are  very  com- 
mon among  certain  of  the  seaweeds,  and  are  found  among 
higher  plants,  as  the  utricularias  or  bladderworts,  which 


^x 


^^ 


^:^ 


Fig.  162.    A  group  of  marine  seaweeds  (Laminarias).    Note  the  various  habits  of  the 
plant  body  and  the  root-like  holdfasts— After  Kerner. 


have  received  their  name  from  the  numerous  bladders 
developed  in  connection  with  their  bodies  (see  Fig.  101), 
and  which  are  also  put  to  additional  uses. 


182  PLANT  STUDIES 

132.  Associations. — The  hydrophyte  associations  may  be 
put  into  two  great  divisions : 

1.  True  hydrophytes,  in  which  the  contents  and  tem- 
perature of  the  water  are  favorable  to  plant  activity. 
Among  such  associations  may  be  mentioned  the  following : 
(1)  Free-swimming  associations,  in  which  the  plants  are 
entirely  sustained  by  water,  as  the  "pond  associations," 
composed  of  algae,  duckweeds,  etc.,  which  float  in  stagnant 
or  slow-moving  waters. 

(2)  Pondweed  associations,  in  which  the  plants  are 
anchored,  but  their  bodies  are  submerged  or  floating. 
Here  belong  the  "  rock  associations,"  consisting  of  plants 
anchored  to  some  firm  support  under  water,  as  the  algae ; 
and  the  "  loose-soil  associations,"  which  imbed  their  roots 
in  the  mucky  soil  of  the  bottom  (Fig.  163),  the  water 
lilies  and  pickerel  weeds  being  conspicuous  illustrations. 

(3)  Swamp  associations,  in  which  the  plants  are  rooted 
in  water,  or  in  soil  rich  in  water,  but  the  leaf-bearing  stems 
rise  above  the  surface.  The  conspicuous  swamp  associa- 
tions are  "  reed  swamps,"  characterized  by  bulrushes,  cat- 
tails, and  reed-grasses  (Figs.  164,  167);  "swamp-moors," 
the  ordinary  swamps,  marshes,  bogs,  etc.,  and  dominated 
by  coarse  sedges  and  grasses  (Fig.  163) ;  and  "  swamp- 
thickets,"  consisting  of  willows,  alders,  birches,  etc. 

2.  Xerophytic  hydrophytes,  in  which  the  contents  and 
temperature  of  the  water  are  unfavorable  to  plant  activity, 
and  the  structures  of  the  plants  are  adapted  to  reduce 
transpiration.  This  results  in  such  xerophytic  structures 
as  are  displayed  by  the  true  xerophytes  (see  §144).  Here 
belong  the  "  sphagnum  moors  "  (Fig.  191),  in  which  sphag- 
num moss  predominates,  and  is  accompanied  by  numerous 
peculiar  orchids,  heaths,  carnivorous  plants,  etc. ;  "  swamp- 
forests,"  where  tamarack,  spruce,  pine,  etc.,  are  the  pre- 
vailing trees ;  "  mangrove  swamps,"  of  the  flat  tropical  sea- 
coasts;  and  "salt  marshes,"  the  extensive  meadow-like  ex- 
panses of  coarse  sedges  and  grasses  near  the  sea-coast. 


13 


Fig.  165. — A  grouj)  of  jxiiuhvftMis.  'I'lio  stoiuB  are  pustaiiird  in  an  erect  position  by 
the  water,  and  the  narrow  leaves  are  expot^ed  to  a  light  whose  intensity  is  dimii> 
Ished  by  passing  through  the  water.— After  Kerner. 


Fig.  166.  Eel  grass  {Vallisneria),  a  common  pondweed  plant.  The  plants  are 
anchored  and  the  foliage  is  submerged.  The  carpel-bearing  flowers  are  carried  to 
the  surface  on  long  stalks  which  allow  a  variable  depth  of  water.  The  stamen- 
bearing  flowers  remain  submerged,  as  indicated  near  the  lower  left  corner,  the 
flowers  breaking  away  and  rising  to  the  surface,  where  they  float  and  efiect  pollina- 
tion.—After  Kerner. 


Fig.  167.  A  reed  swamp,  fringing  the  low  shore  of  a  lake  or  a  sluggish  stream.  The 
plants  are  tall  and  wand-like,  and  all  are  monocotyls.  Three  types  are  prominent, 
the  reed  grasses  (the  tallest),  the  cat-tails  (at  the  right),  and  the  bulrushes  (a  group 
standing  out  in  deeper  water  near  the  middle  of  the  fringing  growth).  The  plant 
in  the  foreground  at  the  extreme  right  is  the  arrow-leaf  (SagitUiria),  recognized 
by  its  characteristic  leaves.— After  Kerner. 


CHAPTER   XIII 

XEROPHYTE   ASSOCIATIONS 

133.  General  character. — Strongly  contrasted  with  the 
hydrophytes  are  the  xerophytes,  which  are  adapted  to  dry 
air  and  soil.  The  xerophytic  conditions  may  be  regarded 
in  general  as  drouth  conditions.  It  is  not  necessary  for 
the  air  and  soil  to  he  dry  throughout  the  year  to  develop 
xerophytic  conditions.  These  conditions  may  be  put  under 
three  heads  :  (1)  possible  drouth,  in  which  a  season  of 
drouth  may  occur  at  irregular  intervals,  or  in  some  seasons 
may  not  occur  at  all ;  (2)  periodic  drouth,  in  which  there 
is  a  drouth  period  as  definite  as  the  winter  period  in  cer- 
tain regions  ;  (3)  perennial  drouth,  in  which  the  dry  con- 
ditions are  constant,  and  the  region  is  distinctly  an  arid 
or  desert  region. 

However  xerophytic  conditions  may  occur,  the  problem 
of  the  plant  is  always  one  of  water  supply,  and  many  strik- 
ing structures  have  been  developed  to  answer  it.  Plants 
in  such  conditions  must  provide,  therefore,  for  two  things : 
(1)  collection  and  retention  of  water,  and  (2)  prevention  of 
its  loss.  It  is  evident  that  in  these  drouth  conditions  the 
loss  of  water  through  transpiration  (see  §26)  tends  to  be 
much  increased.  This  tendency  in  the  presence  of  a  very 
meager  water  supply  is  a  menace  to  the  life  of  the  plant, 
for  it  is  impossible  to  stop  transpiration  entirely,  as  it 
must  take  place  so  long  as  the  plant  is  alive.  The  adapta- 
tions on  the  part  of  the  plant,  therefore,  are  directed 
towards  the  regulation  of  transpiration,  that  it  may  occur 
188 


XEROPHYTE   ASSOCIATIONS  189 

sufficiently  for  the  life-processes,  but  that  it  may  not  be 
wasteful  to  the  point  of  danger. 

The  regulation  of  transpiration  may  be  accomplished 
in  two  general  ways.  It  will  be  remembered  that  the 
amount  of  transpiration  holds  some  relation  to  the 
amount  of  leaf  exposure  or  exposure  of  green  tissue. 
Therefore,  if  the  amount  of  leaf  exposure  be  diminished, 
the  total  amount  of  transpiration  will  be  reduced.  Another 
general  way  for  regulating  transpiration  is  to  jDrotect 
the  exposed  surface  in  some  way  so  that  the  water  does 
not  escape  so  easily.  In  a  word,  therefore,  the  general 
method  is  to  reduce  the  extent  of  exposed  surface  or  to 
protect  it.  It  must  be  understood  that  plants  do  not  differ 
from  each  other  in  adopting  one  or  the  other  of  these 
methods,  for  both  are  very  commonly  used  by  the  same 
plant. 

Adaptations 

134.  Complete  desiccation. — Some  plants  have  a  very  re- 
markable power  of  completely  drying  up  during  the  drouth 
period,  and  then  reviving  upon  the  return  of  moisture. 
This  power  is  strikingly  illustrated  among  the  lichens  and 
mosses,  some  of  which  can  become  so  dry  that  they  may  be 
crumbled  into  powder,  but  revive  when  moisture  reaches 
them.  A  group  of  club  mosses,  popularly  known  as  "  res- 
urrection plants,''  illustrates  this  same  power.  The  dried 
up  nest-like  bodies  of  these  plants  are  common  in  the 
markets,  and  when  they  are  placed  in  a  bowl  of  water  they 
expand  and  may  renew  their  activity.  In  such  cases  it  can 
hardly  be  said  that  there  is  any  special  effort  on  the  part  of 
the  plant  to  resist  drouth,  for  it  seems  to  yield  completely 
to  the  dry  conditions  and  loses  its  moisture.  The  power 
of  reviving,  after  being  completely  dried  out,  is  an  offset, 
however,  for  protective  structures. 

135.  Periodic  reduction  of  surface. — In  regions  of  periodic 


190 


PLANT   STUDIES 


drouth  it  is  very  com- 
mon for  plants  to 
diminish  the  exposed 
surface  in  a  very  de- 
cided way.  In  such 
cases  there  is  what 
may  be  called  a  peri- 
odic surface  decrease. 
For  example,  annual 
plants  remarkably 
diminish  their  ex- 
posed surface  at  the 
period  of  drouth  by 
being  represented 
only  by  well-pro- 
tected seeds.  The 
whole  exposed  sur- 
face of  the  plant, 
root,  stem,  and  leaves, 
has  disappeared,  and 
the  seed  preserves  the 
plant  through  the 
drouth. 

Little  less  remark- 
able is  the  so-called 
geophilous  habit.  In 
this  case  the  whole  of 
the  plant  surface  ex- 
posed to  the  air  dis- 
appears, and  only 
underground  parts, 
such  as  bulbs,  tubers, 
etc.,  persist  (see  Figs. 
45,  46,  66,  67,  68, 
69,  70,  75,  144,  168, 
169).       At    the    re- 


FiG.  168.  The  bloodroot  (San giii7i aria),  showing 
the  subterranean  rootstock  sending  leaves  and 
flower  above  the  surface.— After  Atkinson. 


XEKOPIIYTE  ASSOCIATIONS 


191 


^(V^ 


Fig.    169.     The  spring 
beauty      {Claytonia), 
showing  subterranean 
tuber-like  stem  sending  leaf  and  flower-bearing 
etem  above  the  surface.— After  Atkinson. 


turn  of  the  moist  season 
these  underground  parts 
develop  new  exposed 
surfaces.  In  such  cases 
it  may  be  said  that  at 
the  coming  of  the  drouth 
the  plant  seeks  a  sub- 
terranean retreat. 

A  little  less  decrease 
of  exposed  surface  is 
shown  by  the  deciduous 
habit.  It  is  known  that 
certain  trees  and  shrubs, 
whose  bodies  remain 
exposed  to  the  drouth, 
shed  their  leaves  and 
thus  very  greatly  reduce 
the  amount  of  exposure  j 
with  the  return  of  mois- 
ture, new  leaves  are  put 
forth.  It  will  be  re- 
marked, in  this  connec- 
tion, that  the  same 
habits  serve  just  as  well 
to  bridge  over  a  period 
of  cold  as  a  period  of 
drouth,  and  perhaps 
they  are  more  familiar 
in  connection  with  the 
cold  period  than  in  con- 
nection with  the  drouth 
period. 

13G.  Temporary  reduc- 
tion of  surface. — AVhile 
the  habits  above  have  to 
do  with  regular  drouth 


X92  PLANT   STUDIES 

periods,  there  are  other  habits  by  which  a  temporary  re- 
duction of  surface  may  be  secured.  For  instance,  at  the 
approach  of  a  period  of  drouth,  it  is  very  easy  to  observe 
certain  leaves  rolling  up  in  various  ways.  As  a  leaf  be- 
comes rolled  up,  it  is  evident  that  its  exposed  surface  is 
reduced.  The  behavior  of  grass  leaves,  under  such  cir- 
cumstances, is  very  easily  noted.  A  comparison  of  the  grass 
blades  upon  a  well-watered  lawn  with  those  upon  a  dried-up 
lawn  will  show  that  in  the  former  case  the  leaves  are  flat, 
and  in  the  latter  more  or  less  rolled  up.  The  same  habit 
is  also  very  easily  observed  in  connection  with  the  larger- 
leaved  mosses,  which  are  very  apt  to  encounter  drouth 
periods. 

137.  Fixed  light  position. — In  general,  when  leaves  have 
reached  maturity,  they  are  unable  to  change  their  position 
in  reference  to  light,  having  obtained  what  is  known  as  a 
fixed  light  position.  During  the  growth  of  the  leaf,  how- 
ever, there  may  be  changes  in  direction  so  that  the  fixed 
light  position  will  depend  upon  the  light  direction  during 
growth.  The  position  finally  attained  is  an  expression  of 
the  attempt  to  secure  sufficient,  but  not  too  much  light 
(see  §13).  The  most  noteworthy  fixed  positions  of  leaves 
are  those  which  have  been  developed  in  intense  light. 
A  very  common  position  in  such  cases  is  the  profile  posi- 
tion, in  which  the  leaf  apex  or  margin  is  directed  upwards, 
and  the  two  surfaces  are  more  freely  exposed  to  the  morn- 
ing and  evening  rays — that  is,  the  rays  of  low  intensity — 
than  to  those  of  midday. 

Illustrations  of  leaves  with  one  edge  directed  upwards 
can  be  obtained  from  the  so-called  compass  plants.  Prob- 
ably most  common  among  these  are  the  rosin-weed  of  the 
prairie  region,  and  the  prickly  lettuce,  which  is  an  intro- 
duced plant  very  common  in  waste  ground  (see  Fig.  170). 
Such  plants  received  their  popular  name  from  the  fact  that 
many  of  the  leaves,  when  edgewise,  point  approximately 
north  and  south,  but  this  direction  is  very  indefinite.     It  is 


XEKOPIIVTE  ASSOCIATION'S 


iy3 


evident  that  such  a  position  avoids  exposure'  of  the  leaf 
surface  to  the  noon  rays,  but  obtains  for  these  same  sur- 
faces the  morning  and  evening  rays.  If  these  plants  are 
developed  in  the  shade,  the   ''  compass "  habit  does  not 


Fig.  170.  Two  compass  plants.  The  two  figures  to  the  left  represent  the  same  plant 
{Silphium)  viewed  from  the  east  and  from  the  south.  The  two  figures  to  the  right 
represent  the  same  relative  positions  of  the  leaves  of  Zae^Mca.— After  Keener. 


appear  (see  §15).  The  profile  position  is  a  very  common 
one  for  the  leaves  of  Australian  plants,  a  fact  which  gives 
much  of  the  vegetation  a  peculiar  appearance.  All  these 
positions  are  serviceable  in  diminishing  the  loss  of  water, 
which  would  occur  with  exposure  to  more  intense  light. 
138.   Motile  leaves. — Although  in  most  plants  the  mature 


194 


PLANT  STUDIES 


leaves  are  in  a  fixed  position,  there  are  certain  ones  whose 
leaves  are  able  to  perform  movements  according  to  the  need. 
Mention  has  been  made  already  of  such  forms  as  Oxalis 
(see  §14),  whose  leaves  change  their  position  readily  in 
reference  to  light.  Motile  leaves  have  been  developed  most 
extensively  among  the  Leguminosce,  the  family  to  which 


Fig.  171.  Two  twigs  of  a  sensitive  plant.  The  one  to  the  left  shows  the  numerous 
small  leaflets  in  their  expanded  position  ;  the  one  to  the  right  shows  the  greatly 
reduced  surface,  the  leaflets  folded  together,  the  main  leaf  branches  having 
approached  one  another,  and  the  main  leaf-stalk  having  bent  sharply  downwards. 
— After  Strasbxjrger. 


belong  peas,  etc.  In  this  family  are  the  so-called  ^^  sen- 
sitive plants,^"  which  have  received  their  popular  name 
from  their  sensitive  response  to  light  as  well  as  to  other 
influences  (see  Fig.  171).  The  acacia  and  mimosa  forms 
are  the  most  notable  sensitive  plants,  and  are  esjDecially 
developed  in  arid  regions.  The  leaves  are  usually  very 
large,  but  are  so  much  branched  that  each  leaf  is  com- 
posed  of  very  numerous  small  leaflets.     Each  leaflet  has 


XEROPIIYTE  ASSOCIATIONS 


195 


the  power  of  independent  motion,  or  the  whole  leaf  may 
move.  If  there  is  danger  from  exposure  to  drouth,  some 
of  the  leaflets  will  be  observed  to  fold  together  ;  in  case 


Fig.  172.    A  heath  plant  (Erica),  showing  low,  bushy  growth  and  small  leaves. 

the  danger  is  prolonged,  more  leaflets  will  fold  together ; 
and  if  the  danger  persists,  the  surface  of  exposure  will  be 
still  further  reduced,  until  the  whole  jolant  may  have  its 
leaves  completely  folded  up.     In  this  way  the  amount  of 


) 


196  PLANT  STUDIES 

reduction  of  the  exposed  surface  may  be  accurately  regu- 
lated to  suit  the  need  (see  §38). 

139.  Reduced  leaves. — In  regions  that  are  rather  per- 
manently dry,  it  is  observed  that  the  plants  in  general  pro- 
duce smaller  leaves  than  in  other  regions  (see  Fig.  173). 
That  this  holds  a  direct  relation  to  the  dry  conditions  is 


Fig.  173.  Leaves  from  the  common  baeswood  (Tilia),  showing  the  effect  of  environ- 
ment ;  those  at  the  right  being  from  a  tree  growing  in  a  river  bottom  (mesophyte 
conditions) ;  those  at  the  left  being  from  a  tree  growing  upon  a  dune,  where  it  is 
exposed  to  intense  light,  heat,  cold,  and  wind.  Not  only  are  the  former  larger, 
but  they  are  much  thinner.  The  leaves  from  the  dune  tree  are  strikingly  smaller, 
much  thicker,  and  more  compact.— After  Cowles. 

evident  from  the  fact  that  the  same  plant  often  produces 
smaller  leaves  in  xerophytic  conditions  than  in  moist  con- 
ditions. One  of  the  most  striking  features  of  an  arid 
region  is  the  absence  of  large,  showy  leaves  (see  Fig.  172). 
These  reduced  leaves  are  of  various  forms,  such  as  the 
needle  leaves  of  pines,  or  the  thread-like  leaves  of  certain 
sedges  and  grasses,  or  the  narrow  leaves  with  inrolled 
margins  such  as  is  common  in  many  heath  plants.     The 


XEKOPllYTE  ASSOCIATIONS 


197 


Fig.  174.    Two  species  of  Achillea  on  different  soils.    The  one  to  the  left  was  grown 
in    drier    conditions    and    shows    an    abundant    development    of    hairs.— After 

SCHIMPER. 


extreme  of  leaf  reduction  has  been  reached  by  the  cactus 
plants,  whose  leaves,  so  far  as  foliage  is  concerned,  have 
disappeared  entirely,  and   the   leaf  work  is   done   by  the 


198 


PLANT   STUDIES 


surface  of  the  globular,  cylindrical,  or  flattened  stems  (see 
§36). 

140.  Hairy  coverings. — A  covering  of  hairs  is  an  effective 
sun  screen,  and  it  is  very  common  to  find  plants  of  xerophyte 
regions  character- 
istically hairy  (see 
§35).  The  hairs 
are  dead  struc- 
tures, and  within 
them  there  is  air. 
This  causes  them 
to  reflect  the  light, 
and  hence  to  ap- 
pear white  or 
nearly  so.  This 
reflection  of  light 
by  the  hairs  dimin- 
ishes the  amount 
which  reaches  the 
working  region  of 
the  plant  (see  Fig. 
174). 

141.  Body  habit. 
— Besides  the  va- 
rious   devices   for 
diminishing   ex- 
posure or  leaf  sur- 
face,   and    hence 
loss   of    water, 
enumerated  above, 
the  whole  habit  of 
the  plant  may  em- 
phasize the  same  purpose.    In  dry  regions  it  is  to  be  observed 
that  dwarf  growths  prevail,  so  that  the  plant  as  a  whole 
does  not  present  such  an  exposure  to  the  dry  air  as  in 
regions  of  greater  moisture  (see  Fig.  175).     Also  the  pros- 


FiG.  175.  Two  plants  of  a  common  scouring  rush  {Equi- 
setum)^  showing  the  effect  of  environment ;  the  long, 
unbranched  one  having  grown  in  normal  mesophyte 
conditions  ;  the  short,  bushy  branching,  more  slender 
form  having  grown  on  the  dunes  (xerophyte  condi- 
tions).—After  COWLES. 


AEKoriliTE  ASS9CIATI0N8 


199 


irate  or  creeping  habit  is  a  much  less  exposed  one  in  such 
regions  than  the  erect  habit.  In  the  same  manner,  the  very 
characteristic  rosette  habit,  with  its  cluster  of  overlapping 
leaves  close  against  the  ground,  tends  to  diminish  loss  of 
water  through  transpiration. 

One  of  the  most  common  results  of  xerophytic  conditions 
upon  body  habit  is  the  development  of  thorns  and  spiny 


Fig.  176. 


Young  plants  of  Euphorbia  splendens,  showing  a  development  of  thorns 
characteristic  of  the  plants  of  dry  regions. 


processes.  As  a  consequence,  the  vegetation  of  dry  regions 
is  characteristically  spiny.  In  many  cases  these  spiny  pro- 
cesses can  be  made  to  develop  into  ordinary  stems  or  leaves 
in  the  presence  of  more  favorable  water  conditions.  It  is 
probable,  therefore,  that  such  structures  represent  reduc- 
tions in  the  growth  of  certain  regions,  caused  by  the  unfavor- 
able conditions.  Incidentally  these  thorns  and  spiny  pro- 
cesses are  probably  of  great  service  as  a  protection  to  plants 
in  regions  where  vegetation  is  peculiarly  exposed  to  the 
14 


200 


PLANT   STUDIES 


ravages  of  animals  (see  §105).  Examine  Figs.  i76,  177, 
178,  179,  180,  181. 

142.  Anatomical  adaptations. — It  is  in  connection  with 
the  xerophytes  that  some  of  the  most  striking  anatomical 
adaptations  have  been 
developed.  In  such 
conditions  the  epider- 
mis is  apt  to  be  cov- 
ered by  layers  of 
cuticle,  which  are  de- 
veloped by  the  walls 
of  the  epidermal  cells, 
and  being  constantly 
formed  beneath,  the 
cuticle  may  become 
very  thick.  This 
forms  a  very  efficient 
protective  covering, 
and  has  a  tendency  to 
diminish  the  loss  of 
water  (see  §35).  It  is 
also  to  be  observed 
that  among  xerophytes 
there  is  a  strong  de- 
velopment of  palisade 
tissue.  The  working 
cells  of  the  leaves  next 
to  the  exposed  surface 
are  elongated,  and  are 
directed     endwise     to 

the  surface.  In  this  way  only  the  ends  of  the  elongated 
cells  are  exposed,  and  as  such  cjells  stand  very  closely  to- 
gether, there  is  no  drying  air  between  them.  In  some 
cases  there  may  be  more  than  one  of  these  palisade  rows 
(see  §32).  It  has  been  observed  that  the  chloroplasts  in 
these  palisade  cells  are  able  to  assume  various  positions  in 


b  d 

Fig.  177.  Two  plants  of  common  gorge  or  furze 
iJJlex)^  showing  the  effect  of  environment  :  h 
is  a  plant  grown  in  moist  conditions ;  a  is  a 
plant  grown  in  dry  conditions,  the  leaves  and 
branches  having  been  almost  entirely  developed 
as  thorns.— After  Lothelier. 


2LEK0PHYTE  ASSOCIATIONS 


201 


Fig.  178.  A  branch  of  Cytisus,  showing  the 
reduced  leaves  and  thorny  branches.— After 
Kerner. 


regulation  of  transpiration,  but 
storage  of  water,  as  it  is  received  at  rare  inter- 
vals. It  is  very  common  to  find  a  certain  re- 
gion of  the  plant  body  given  over  to  this  work, 
forming  what  is  known  as  water  tissue.  In 
many  leaves  this  water  tissue  may  be  distin- 
guished from  the  ordinary  working  cells  by 
being  a  group  of  colorless  cells  (see  Fig.  183). 
In  plants  of  the  drier  regions  leaves  may 
become  thick  and  fleshy  through  acting  as 
water  reservoirs,  as  in  the  case  of  the  agave, 
sedums,  etc.  Fleshy  or  "  succulent "  leaves 
are  regarded  as  adaptations  of  prime  impor- 


the  cell,  so  that  when 
the  light  is  very  intense 
they  move  to  the  more 
shaded  depths  of  the 
cell,  and  when  it  be- 
comes less  intense  they 
move  to  the  more  ex- 
ternal regions  of  the 
cell  (see  Fig.  182). 
The  stomata,  or  air 
pores,  which  are  devel- 
oped in  the  epidermis, 
are  also  great  regulators 
of  transpiration,  as  has 
been  mentioned  already 
(see  §31). 

143.  Water  reservoirs. 
— In  xero- 
phytes  at- 
t  e  n  t  i  0  n 
must  be 
given  not 
only  to  the 
also   to   the 


Fig.  179.  A 
leaf  of  traga- 
canth,  show- 
ing the  re- 
duced leaf- 
lets and  the 
thorn-like 
tip.— After 
Kerner. 


202 


PLANT   STUDIES 


tance  in  xerophytic  conditions.  In 
the  cactus  plants  the  peculiar  stems 
have  become  great  reservoirs  of 
moisture.  The  globular  body  may 
be  taken  to  represent  the  most  com- 
plete answer  to  this  general  problem, 
as  it  is  the  form  of  body  by  which 
the  least  amount  of  surface  may  be 
exposed  and  the  greatest  amount  of 
water  storage  secured.  In  the  case 
of  fleshy  leaves  and  fleshy  bodies  it 
has  long  been  noticed  that  they  not 
only  contain  water,  but  also  have  a 
great  power  of  re- 


FiG.  180.  A  fragment  of  bar- 
berry, showing  the  thorns. 
— After  Kerner. 


Fig.  181.  Twig  of  com- 
mon locust,  showing 
the  thorns.— After 
Kerner. 


taining  it.  Plant 
collectors  have  found  much  difficulty  in 
drying  these  fleshy  forms,  some  of  which 
seem  to  be  able  to  retain  their  moisture  in- 
definitely, even  in  the  driest  conditions. 
144.  Xerophytic  structure. — The  adap- 
tations given  above  are  generally  found 
in  plants  growing  in  drouth  conditions, 
and  they  all  imply  an  effort  to  diminish 
transpiration.  It  must  not  be  supposed, 
however,  that  only  plants  living  in 
drouth  conditions  show  these  adaj^ta- 
tions.  Such  adaptations  result  in  what 
is  known  as  the  xerophytic  structure, 
and  such  a  structure  may  appear  even 
in  2)lants  growing  in  hydrophyte  condi- 
tions. For  example,  the  bulrush  grows 
in  shallow  water,  and  is  a  prominent 
member  of  one  of  the  hydrophyte  asso- 
ciations (see  §132) ;  and  yet  it 'has  a  re- 
markably xero^Dhytic  structure.  This  is 
probably  due  to  the  fact  that  although  it 


XEROrilYTE   ASSOCIATIONS 


stands  in  the  water  its  stem  is  exposed 
to  a  heat  which  is  often  intense. 

The  ordinary  prairie  (see  §146)  is 
included  among  mesophyte  associa- 
tions on  account  of  the  rich,  well- 
watered  soil;  and  yet  many  of  the 
plants  are  very  xerophytic  in  struc- 
ture, probably  on  account  of  the  pre- 
vailing dry  winds. 

The  ordinary  sphagnum-bog  (see 
§132),  or  "peat-bog,"  is  included 
among  hydrophyte  associations.  It 
has  an  abundance  of  water,  and  is  not 
exposed  to  blazing  heat,  as  in  the  case 
of  the  bulrushes,  or  to  drying  wind., 
as  in  the  case  of  prairie  plants ;  and 
yet  its  plants  show  a  xerophytic  struc- 
ture. The  cause  for  this  has  not  yet 
been  determined,  although  several 
suggestions  have  been  made. 

It  is  evident,  therefore,  that  xero- 
phytic structures  are  not  necessarily 
confined  to  xerophytic  situations.  It 
is  probably  true  that  all  associations 
which  show  xerophytic 
structures  belong  to- 
gether more  naturally 
than  do  the  associa- 
tions which  are 
grouped  according  to 
the  water  supjily. 

Associatio?is 

No  attempt  will  be        Fi«.  183.     a  eection  through  a  i?f{70«ia  leaf,  show- 
made    to    classify  these  '"S  the  epidermis  (ep)  above  and  below,  the 

water-storage  tissue  (tvs)  above  and  below,  and 
very  numerous  aSSOCia-  the  central  chlorophyll  region  (as). 


Fig.  182.  Cells  from  the  leaf 
of  a  quillwort  (Isoetes). 
The  light  is  striking  the 
cells  from  the  direction  of 
one  looking  at  the  illus- 
tration. If  it  be  some- 
what diffuse  the  cliloro- 
plasts  distribute  them- 
selves through  the  shal- 
low cell,  as  in  the  cell  to 
the  left.  If  the  light  be 
intense,  the  chloroplasts 
move  to  the  wall  and  as- 
sume positions  less  ex- 
posed, as  in  the  cell  to 
the  right. 


204: 


PLANT  STUDIES 


tiotis,  but  a  few  prominent  illustrations  will  be  given.  Some 
of  the  prominent  associations  are  as  follows :  "  rock  associa- 
tions," composed  of  plants  living  upon  exposed  rock  surfaces, 
etc.,  notably  lichens  and  mosses  (Fig.  184) ;  "  sand  associa- 
tions," including  beaches,  dunes  (Fig.  185),  etc. ;  "shrubby 
heaths,"  characterized  by  heath  plants  ;  "  plains,"  the  great 
areas  with  dry  air  developed  in  the  interiors  of  continents 
(Fig.  186);  "  cactus  deserts,"  still  more  arid  areas  of  the  Mex- 
ican region,  where  the  cactus,  agave,  etc.,  have  learned  to  live 


1 

■%.a^^>^''^-^ 

^"^ 

l:-S^A\W 

.::Ji^.^#i 

Fig.  184.    A  rock  covered  with  lichens. 


(Fig.  190) ;  "  tropical  deserts,"  where  xerophytic  condi- 
tions reach  their  extreme  in  the  combination  of  maximum 
heat  and  minimum  water  ;  "  xerophyte  thickets,"  the  most 
impenetrable  of  all  thicket-growths,  represented  by  the 
"chaparral"  of  the  southwest  (Fig.  187),  and  the  "bush" 
of  Africa  and  Australia ;  "  xerophyte  forests,"  also  notably 
coniferous.     (See  Figs.  192,  193.) 


Fig.  189.  Two  plants  of  the  giant  cactus.  Note  the  fluted,  clumsy  branching,  leaf- 
less bodies  growing  from  the  rocky,  sterile  soil  characteristic  of  cactus  deserts. 
Certain  dry-ground  grasses  and  low,  shrubby  plants  with  small  leaves  may  be  seen 
in  the  foreground. 


I 


Fi'j.  192.— A  xerophyte  conifer  forest  in  the  Cumberland    Mountains    of    Tennessee. 
The  table  mountain  pines  find  footholds  in  crevices  of  the  rocks. 


CHAPTER  XIV 

MESOPHYTE    ASSOCIATIONS 

145.  General  characters. — Mesophytes  make  up  the  com- 
mon vegetation  of  temperate  regions,  the  vegetation  most 
commonly  met  and  studied.  The  conditions  of  moisture 
are  medium,  precipitation  is  in  general  evenly  distributed, 
and  the  soil  is  rich  in  humus.  The  conditions  are  not  ex- 
treme, and  therefore  special  adaptations,  such  as  are  neces- 
sary for  xerophyte  or  hydrophyte  conditions,  do  not  appear. 
This  may  be  regarded  as  the  normal  plant  condition.  It 
is  certainly  the  arable  condition,  and  most  adapted  to  the 
plants  Avhich  men  seek  to  cultivate.  When  for  purposes 
of  cultivation  xerophyte  areas  are  irrigated,  or  hydrophyte 
areas  are  drained,  it  is  simply  to  bring  them  into  mesophy  te 
conditions. 

In  looking  over  a  mesophyte  area  and  contrasting  it 
with  a  xerophyte  area,  one  of  the  first  things  evident  is  that 
the  former  is  far  richer  in  leaf  forms.  It  is  in  the  meso- 
phyte conditions  that  foliage  leaves  show  their  remarkable 
diversity.  In  hydrophyte  and  xerophyte  areas  they  are  apt 
to  be  more  or  less  monotonous  in  form.  Another  contrast 
is  found  in  the  dense  growth  over  mesophyte  areas,  much 
more  so  than  in  xerophyte  regions,  and  even  more  dense 
than  in  hydrophyte  areas. 

Among  the  mesophyte  associations  must  be  included  not 

merely  the  natural  ones,  but  those  new  associations  which 

have  been  formed  under  the  influence  of  man,  and  which  do 

not  appear  among  xerophyte  and  hydrophyte  associations. 

214 


MESOPHYTE   ASSOCIATIONS  215 

These  new  associations  have  been  formed  by  the  introduc- 
tion of  weeds  and  culture  plantSc 

140.  The  two  groups  of  associations. — Two  very  prom- 
inent types  of  associations  are  included  here  under  the 
mesophytes,  although  they  are  probably  as  distinct  from  one 
another  as  are  the  mesophyte  and  xerophyte  associations. 
One  group  is  composed  of  low  vegetation,  notably  the  com- 
mon grasses  and  herbs ;  the  other  is  a  higher  woody  vegeta- 
tion, composed  of  shrubs  and  trees.  The  most  characteristic 
types  under  each  one  of  these  divisions  are  noted  as  follows. 

Among  the  mesophyte  grass  and  herb  associations  are 
the  *'  arctic  and  alpine  carpets,"  so  characteristic  of  high 
latitudes  and  altitudes  where  the  conditions  forbid  trees, 
shrubs,  or  even  tall  herbs  ;  "  meadows,"  areas  dominated  by 
grasses  (Fig.  197),  the  prairies  being  the  greatest  meadows,, 
where  grasses  and  flowering  herbs  are  richly  displayed  (Fig. 
198) ;  "  pastures,"  drier  and  more  open  than  meadows. 

Among  the  woody  mesophyte  associations  are  the  "  thick- 
ets," composed  of  willow,  alder,  birch,  hazel,  etc.,  either 
pure  or  forming  a  jungle  of  mixed  shrubs,  brambles,  and 
tall  herbs;  "deciduous  forests/'  the  glory  of  the  temperate 
regions,  rich  in  forms  and  foliage  display,  with  annual  fall 
of  leaves,  and  exhibiting  the  remarkable  phenomenon  of 
autumnal  coloration  (Figs.  194-196);  "rainy  tropical  for- 
ests," in  the  region  of  trade  winds,  heavy  rainfalls,  and 
great  heat,  where  the  world's  vegetation  reaches  its  climax, 
and  where  in  a  saturated  atmosphere  gigantic  jungles  are 
developed,  composed  of  trees  of  various  heights,  shrubs  of 
all  sizes,  tall  and  low  herbs,  all  bound  together  in  an  inex- 
tricable tangle  by  great  vines  or  lianas,  and  covered  by  a 
luxuriant  growth  of  numerous  epiphytes  (Fig.  199). 


16 


CHAPTER  XV 

THE  PLANT  GROUPS 

147.  Differences  in  structure. — It  is  evident,  even  to  the 
casual  observer,  that  plants  differ  very  much  in  structure. 
They  differ  not  merely  in  form  and  size,  but  also  in  com- 
plexity. Some  plants  are  simple,  others  are  complex,  and 
the  former  are  regarded  as  of  lower  rank.  For  example, 
a  lichen,  a  moss,  and  an  oak  differ  very  much  in  form  and 
size^  and  also  in  complexity,  and  because  of  this  last  fact  an 
oak  would  be  regarded  as  a  plant  of  higher  rank  than  either 
a  lichen  or  a  moss.  It  must  not  be  supposed  that  rank  is 
measured  by  size,  for  in  the  highest  group  there  are  many 
small  plants. 

Beginning  with  the  simplest  plants — that  is,  those  of 
lowest  rank — one  can  pass  by  almost  insensible  grada- 
tions to  those  of  highest  rank.  At  certain  points  in  this 
advance  notable  interruptions  of  the  continuity  are  dis- 
covered, structures,  and  hence  certain  habits  of  work,  chang- 
ing decidedly,  and  these  breaks  enable  one  to  organize  the 
vast  array  of  plants  into  groups.  Some  of  the  breaks  ap- 
pear to  be  more  important  than  others,  and  opinions  may 
differ  as  to  those  of  chief  importance,  but  it  is  customary 
to  select  three  of  them  as  indicating  the  division  of  the 
plant  kingdom  into  four  great  groups. 

148.  The  great  groups. — Tlie  four  great  groups  may  be 
indicated  here,  but  it  must  be  remembered  that  their  names 
mean  nothing  until  plants  representing  tliem  have  been 
studied.     It  will  be  noticed  that  all  the  names  have  the 

221 


222  PLANT  STUDIES 

constant  termination  phytes,  which  is  a  Greek  word  mean- 
ing "  plants."  The  prefix  in  each  case  is  also  a  Greek  word 
intended  to  indicate  the  kind  of  plants. 

(1)  Thallophytes. — The  name  means  "thallus  plants," 
hut  just  what  a  "thallus"  is  can  not  well  be  explained 
until  some  of  the  plants  have  been  examined.  In  this 
great  group  are  included  some  of  the  simplest  forms, 
known  as  AlgcB  and  Fungi^  the  former  represented  by  green 
thready  growths  in  fresh  water  and  the  great  host  of  sea- 
weeds, the  latter  by  moulds,  mushrooms,  etc. 

(2)  Bryophytes. — The  name  means  "  moss  plants,"  and 
suggests  very  definitely  the  forms  which  are  included. 
Every  one  knows  mosses  in  a  general  way,  but  associated 
with  them  in  this  great  group  are  the  allied  liverworts, 
which  are  very  common  but  not  so  generally  known. 

(3)  Pteridophytes. — The  name  means  "  fern  plants,"  and 
ferns  are  well  known.  Not  all  Pteridophytes,  however,  are 
ferns,  for  associated  with  them  are  the  horsetails  (scouring 
rushes)  and  the  club  mosses. 

(4)  Spermatopliytes. — The  name  means  "  seed  plants  " — 
that  is,  those  plants  which  produce  seeds.  In  a  general 
way  these  are  the  most  familiar  plants,  and  are  commonly 
spoken  of  as  "  flowering  plants."  They  are  the  highest  in 
rank  and  the  most  conspicuous,  and  hence  have  received 
much  attention.  In  former  times  the  study  of  botany  in 
the  schools  was  restricted  to  the  examination  of  this  one 
group,  to  the  entire  neglect  of  the  other  three  great  groups. 

149.  Increasing  complexity. — At  the  very  outset  it  is  well 
to  remember  that  the  Thallophytes  contain  the  simplest 
plants — those  whose  bodies  have  developed  no  organs  for 
special  work,  and  that  as  one  advances  through  higher 
Thallophytes,  Bryophytes,  and  Pteridophytes,  there  is  a  con- 
stant increase  in  the  complexity  of  the  plant  body,  until  in 
the  Spermatophytes  it  becomes  most  highly  organized,  with 
numerous  structures  set  apart  for  special  work,  just  as  in  the 
highest  animals  limbs,  eyes,  ears,  bones,  muscles,  nerves,  etc., 


THE   PLANT   GROUPS  223 

are  set  apart  for  special  work.  The  increasing  complexity 
is  usually  spoken  of  as  differentiation — that  is,  the  setting 
apart  of  structures  for  different  kinds  of  work.  Hence  the 
Bryophytes  are  said  to  be  more  highly  differentiated  than 
the  Thallophytes,  and  the  Spermatophytes  are  regarded  as 
the  most  highly  differentiated  group  of  plants. 

150.  Nutrition  and  reproduction. — However  variable  plants 
may  be  in  complexity,  they  all  do  the  same  general  kind  of 
work.  Increasing  complexity  simply  means  an  attempt  to 
do  this  work  more  effectively.  It  is  plant  work  that  makes 
plant  structures  significant,  and  hence  in  this  book  no  at- 
tempt will  be  made  to  separate  them.  All  the  work  of 
plants  may  be  put  under  two  heads,  nutrition  and  repro- 
duction^ the  former  including  all  those  processes  by  which 
a  plant  maintains  itself,  the  latter  those  processes  by  which 
it  produces  new  plants.  In  the  lowest  plants,  these  two 
great  kinds  of  work,  or  functions^  as  they  are  called,  are 
not  set  apart  in  different  regions  of  the  body,  but  usually 
the  first  step  toward  differentiation  is  to  set  apart  the  re- 
productive function  from  the  nutritive,  and  to  develop 
special  reproductive  organs  which  are  entirely  distinct  from 
the  general  nutritive  body. 

151.  The  evolution  of  plants. — It  is  generally  supposed  that 
the  more  complex  plants  have  descended  from  the  simpler 
ones ;  that  the  Bryophytes  have  been  derived  from  the  Thallo- 
phytes, and  so  on.  All  the  groups,  therefore,  are  supposed 
to  be  related  among  themselves  in  some  way,  and  it  is  one 
of  the  great  problems  of  botany  to  discover  these  relation- 
ships. This  theory  of  the  relationship  of  plant  groups  is 
known  as  the  theory  of  descent^  or  more  generally  as  evo- 
lution. To  understand  any  higher  group  one  must  study 
the  lower  ones  related  to  it,  and  therefore  the  attempt  of 
this  book  will  be  to  trace  the  evolution  of  the  plant  king- 
dom, by  beginning  with  the  simplest  forms  and  notins:  the 
gradual  increase  in  complexity  until  the  highest  forms  are 
reached. 


CHAPTEK  XVI 

THALLOPHYTES:  ALG^ 

152.  General  characters. — Thallophytes  are  the  simplest  of 
plants,  often  so  small  as  to  escape  general  observation,  but 
sometimes  with  large  bodies.  They  occur  everywhere  in 
large  numbers,  and  are  of  special  interest  as  representing 
the  beginnings  of  the  plant  kingdom.  In  this  group  also 
there  are  organized  all  of  the  principal  activities  of  plants, 
so  that  a  study  of  Thallophytes  furnishes  a  clew  to  the 
structures  and  functions  of  the  higher,  more  complex 
groups. 

The  word  "thallus"  refers  to  the  nutritive  body,  or 
vegetative  body,  as  it  is  often  called.  This  body  does  not 
differentiate  special  nutritive  organs,  such  as  the  leaves  and 
roots  of  higher  plants,  but  all  of  its  regions  are  alike.  Its 
natural  position  also  is  not  erect,  but  prone.  While  most 
Thallophytes  have  thallus  bodies,  in  some  of  them,  as  in 
certain  marine  forms,  the  nutritive  body  differentiates  into 
regions  which  resemble  leaves,  stems,  and  roots  ;  also  cer- 
tain Bryophytes  have  thallus  bodies.  The  thallus  body, 
therefore,  is  not  always  a  distinctive  mark  of  Thallophytes, 
but  must  be  supplemented  by  other  characters  to  determine 
the  group. 

153.  Algae  and  Fungi. — It  is  convenient  to  separate  Thallo- 
phytes into  two  great  divisions,  known  as  Algm  and  Fungi. 
It  should  be  known  that  this  is  a  very  general  division  and 
not  a  technical  one,  for  there  are  groups  of  Thallophytes 
which  can  not  be  regarded  as  strictly  either  Algae  or  Fungi, 
but  for  the  present  these  groups  may  be  included. 
224 


THALLOPHYTES:   ALGM  225 

The  great  distinction  between  these  two  divisions  of 
Thallophytes  is  that  the  Algae  contain  chlorophyll  and  the 
Fungi  do  not.  Chlorophyll  is  the  characteristic  green  color- 
ing matter  found  in  plants,  the  word  meaning  "  leaf  green." 
It  may  be  thought  that  to  use  this  coloring  material  as  the 
basis  of  such  an  important  division  is  somewhat  superficial, 
but  it  should  be  known  that  the  presence  of  chlorophyll  gives 
a  peculiar  power — one  which  affects  the  whole  structure 
of  the  nutritive  body  and  the  habit  of  life.  The  presence 
of  chlorophyll  means  that  the  plant  can  make  its  own  food, 
can  live  independent  of  other  plants  and  animals.  Alg^e, 
therefore,  are  the  independent  Thallophytes,  so  far  as  their 
food  is  concerned,  for  they  can  manufacture  it  out  of  the 
inorganic  materials  about  them. 

The  Fungi,  on  the  other  hand,  contain  no  chlorophyll, 
can  not  manufacture  food  from  inorganic  material,  and 
hence  must  obtain  it  already  manufactured  by  plants  or 
animals.  In  this  sense  they  are  dependent  upon  other  or- 
ganisms, and  this  dependence  has  led  to  great  changes  in 
structure  and  habit  of  life. 

It  is  supposed  that  Fungi  have  descended  from  Algse — 
that  is,  that  they  were  once  Algge,  which  gradually  acquired 
the  habit  of  obtaining  food  already  manufactured,  lost  their 
chlorophyll,  and  became  absolutely  dependent  and  more  or 
less  modified  in  structure.  Fungi  may  be  regarded,  there- 
fore, as  reduced  relatives  of  the  Alg^e,  of  equal  rank  so  far 
as  birth  and  structure  go,  but  of  very  different  habits. 

ALG^ 

154.  General  characters. — As  already  defined.  Algae  are 
Thallophytes  whicli  contain  chlorophyll,  and  are  therefore 
able  to  manufacture  food  from  inorganic  material.  They 
are  known  in  general  as  "seaweeds,"  although  there  are 
fresh-water  forms  as  well  as  marine.  They  are  exceedingly 
variable  in  size,  ranging  from  forms  visible  only  by  means 


226  PLANT   STUDIES 

of  the  compound  microscope  to  marine  forms  with  enor- 
mously bulky  bodies.  In  general  they  are  hydrophytes — 
that  is,  plants  adapted  to  life  in  water  or  in  very  moist 
places.  The  special  interest  connected  with  the  group  is 
that  it  is  supposed  to  be  the  ancestral  group  of  the  plant 
kingdom — the  one  from  which  the  higher  groups  have  been 
more  or  less  directly  derived.  In  this  regard  they  differ 
from  the  Fungi,  which  are  not  supposed  to  be  responsible 
for  any  higher  groups. 

155.  The  subdivisions. — Although  all  the  Algae  contain 
chlorophyll,  some  of  them  do  not  appear  green.  In  some 
of  them  another  coloring  matter  is  associated  with  the  chlo- 
rophyll and  may  mask  it  entirely.  Advantage  is  taken  of 
these  color  associations  to  separate  Algae  into  subdivisions. 
As  these  colors  are  accompanied  by  constant  differences  in 
structure  and  work,  the  distinction  on  the  basis  of  colors  is 
more  real  than  it  might  appear.  Upon  this  basis  four  sub- 
divisions may  be  made.  The  constant  termination  ])hyce(B^ 
which  appears  in  the  names,  is  a  Greek  word  meaning  "  sea- 
weed," which  is  the  common  name  for  Algae ;  while  the  pre- 
fix in  each  case  is  the  Greek  name  for  the  color  which  char- 
acterizes the  group. 

The  four  subdivisions  are  as  follows  :  (1)  CyanophycecB^ 
or  "  Blue  Algae,"  but  usually  called  "  Blue-green  Algse,"  as  the 
characteristic  blue  does  not  entirely  mask  the  green,  and 
the  general  tint  is  bluish-green ;  (2)  Chlorophycece^  or  "  Green 
Algae,"  in  which  there  is  no  special  coloring  matter  associ- 
ated with  the  chlorophyll ;  (3)  PhcsophycecB,  or  "  Brown 
Algae  " ;  and  (4)  Rhodophycece^  or  "  Red  Algae." 

It  should  be  remarked  that  probably  the  Cyanophyceae 
do  not  belong  with  the  other  groups,  but  it  is  convenient  to 
present  them  in  this  connection. 

156.  The  plant  body. — By  this  phrase  is  meant  the  nutri- 
tive or  vegetative  body.  There  is  in  plants  a  unit  of  struc- 
ture known  as  the  cell.  The  bodies  of  the  simplest  plants 
consist  of  but  one  cell,  while  the  bodies  of  the  most  com- 


THALLOPHYTES:    ALG^ 


227 


plex  plants  consist  of  very  many  cells.  It  is  necessary  to 
know  something  of  the  ordinary  living  plant  cell  before 
the  bodies  of  Algae  or  any  other  plant  bodies  can  be  under- 
stood. 

Such  a  cell  if  free  is  approximately  spherical  in  outline 
(Fig.  20i),  but  if  pressed  upon  by  contiguous  cells  may  be- 
come variously  modified  in 
form  (Fig.  200).  Bounding 
it  there  is  a  thin,  elastic 
wall,  composed  of  a  sub- 
stance called  cellulose.  The 
cell  wall,  therefore,  forms  a 
delicate  sac,  which  contains 
the  living  substance  known 
as  inotoiylasm.  This  is  the 
substance  which  manifests 
life,  and  is  the  only  sub- 
stance in  the  plant  which 
is  alive.  It  is  the  proto- 
plasm which  has  organized 
the  cellulose  wall  about  it- 
self, and  which  does  all  the 
plant  work.  It  is  a  fluid 
substance  which  varies  much  in  its  consistence,  sometimes 
being  a  thin  viscous  fluid,  like  the  white  of  an  ^gg^  some- 
times much  more  dense  and  compactly  organized. 

The  protoplasm  of  the  cell  is  organized  into  various 
structures  which  are  called  organs  of  the  cell^  each  organ 
having  one  or  more  special  functions.  One  of  the  most  con- 
spicuous organs  of  the  living  cell  is  the  single  nucleus.,  a  com- 
paratively compact  and  usually  spherical  protoplasmic  body, 
and  generally  centrally  placed  within  the  cell  (Fig.  200). 
All  about  the  nucleus,  and  filling  up  the  general  cavity 
within  the  cell  wall,  is  an  organized  mass  of  much  thinner 
protoplasm,  known  as  cytoplasm.  The  cytoplasm  seems  to 
form  the  general  background  or  matrix  of  the  cell,  and  the 


Fig.  200.  Cells  from  a  moss  leaf,  showing 
nucleus  (B)  in  which  there  is  a  nucle- 
olus, cj'toplasm  (C),  and  chloroplasts 
(.1).— Caldwell. 


228  PLANT   STUDIES 

nucleus  lies  imbedded  within  it  (Fig.  200).  Every  working 
cell  consists  of  at  least  cytoplasm  and  nucleus.  Sometimes 
the  cellulose  wall  is  absent,  and  the  cell  then  consists  sim- 
ply of  a  nucleus  with  more  or  less  cytoplasm  organized 
about  it,  and  is  said  to  be  naked. 

Another  protoplasmic  organ  of  the  cell,  very  conspicuous 
among  the  Algge  and  other  groups,  is  the  plastid.  Plastids 
are  relatively  compact  bodies,  commonly  spherical,  variable 
in  number,  and  lie  imbedded  in  the  cytoplasm.  There  are 
various  kinds  of  plastids,  the  most  common  being  the  one 
which  contains  the  chlorophyll  and  hence  is  stained  green. 
The  chlorophyll-containing  plastid  is  known  as  the  chloro- 
plastid^  or  chlorojplast  (Fig.  200).  An  ordinary  alga-cell, 
therefore,  consists  of  a  cell  wall,  within  which  the  proto- 
plasm is  organized  into  cytoplasm,  nucleus,  and  chloroplasts. 

The  bodies  of  the  simplest  Algae  consist  of  one  such 
cell,  and  it  may  be  regarded  as  the  simplest  form  of  plant 
body.  Starting  with  such  forms,  one  direction  of  advance 
in  complexity  is  to  organize  several  such  cells  into  a  loose 
row,  which  resembles  a  chain  (Fig.  202) ;  in  other  forms 
the  cells  in  a  row  become  more  compacted  and  flattened, 
forming  a  simple  filament  (Fig.  203) ;  in  still  other  forms 
the  original  filament  puts  out  branches  like  itself,  produc- 
ing a  branching  filament  (Fig.  207).  These  filamentous 
bodies  are  very  characteristic  of  the  Alg«. 

Starting  again  with  the  one-celled  body,  another  line  of 
advance  is  for  several  cells  to  organize  in  two  directions, 
forming  a  jt?/«?^e  of  cells.  Still  another  line  of  advance  is  for 
the  cells  to  organize  in  three  directions,  forming  a  mass  of 
cells. 

The  bodies  of  Algae,  therefore,  may  be  said  to  be  one- 
celled  in  the  simplest  forms,  and  in  the  most  complex  forms 
they  become  filaments,  plates,  or  masses  of  cells. 

157.  Reproduction. — In  addition  to  the  work  of  nutrition, 
the  plant  body  must  organize  for  reproduction.  Just  as  the 
nutritive  body  begins  in  the  lowest  forms  with  a  single  cell 


I 


THALLOPHYTES:    ALGJE  229 

and  becomes  more  complex  in  the  higher  forms,  so  repro- 
duction begins  in  very  simple  fashion  and  gradually  be- 
comes more  complex.  Two  general  types  of  reproduction 
are  employed  by  the  Algse,  and  all  other  plants.  They  are 
as  follows : 

(1)  Vegetative  multiplication. — This  is  the  only  type  of 
reproduction  employed  by  the  lowest  Algae,  but  it  persists 
in  all  higher  groups  even  when  the  other  method  has  been 
introduced.  In  this  type  no  special  reproductive  bodies  are 
formed,  but  the  ordinary  vegetative  body  is  used  for  the 
purpose.  For  example,  if  the  body  consists  of  one  cell,  that 
cell  cuts  itself  into  two,  each  half  grows  and  rounds  off  as 
a  distinct  cell,  and  two  new  bodies  appear  where  there  was 
one  before  (Fig.  204).  This  process  of  cell  division  is  very 
complicated  and  important,  involving  a  division  of  nucleus 
and  cytoplasm  so  that  the  new  cells  may  be  organized  just 
as  was  the  old  one.  Wherever  ordinary  nutritive  cells  are 
used  directly  to  produce  new  plant  bodies  the  process  is 
vegetative  mult ij^licat ion.  This  method  of  reproduction  may 
be  indicated  by  a  formula  as  follows  :  P  —  P  —  P  —  P  —  P,  in 
which  P  stands  for  the  plant,  the  formula  indicating  that 
a  succession  of  plants  may  arise  directly  from  one  another 
without  the  interposition  of  any  special  structure. 

(2)  Spores. — Spores  are  cells  which  are  specially  organ- 
ized to  reproduce,  and  are  not  at  all  concerned  in  the  nutri- 
tive work  of  the  plant.  Spores  are  all  alike  in  their  power 
of  reproduction,  but  they  are  formed  in  two  very  distinct 
ways.  It  must  be  remembered  that  these  two  types  of 
spores  are  alike  in  power  but  different  in  origin. 

Asexual  spores. — These  cells  are  formed  by  cell  divi- 
sion. A  cell  of  the  plant  body  is  selected  for  the  purpose, 
and  usually  its  contents  divide  and  form  a  variable  number 
of  new  cells  within  the  old  one  (Fig.  205,  B).  These  new 
cells  are  asexual  spores.,  and  the  cell  which  has  formed  them 
within  itself  is  known  as  the  mother  cell.  This  peculiar 
kind  of  cell  division,  which  does  not  involve  the  wall  of  the 


230 


PLANT  STUDIES 


old  cell,  is  often  called  internal  division^  to  distinguish  it 
from  fission^  which  involves  the  wall  of  the  old  cell,  and  is 
the  ordinary  method  of  cell  division  in  nutritive  cells. 

If  the  mother  cell  which  produces  the  spores  is  different 
from  the  other  cells  of  the  plant  body  it  is  called  the  sporan- 
gium^ which  means  "  spore  vessel."  Often  a  cell  is  nutri- 
tive for  a  time  and  afterward  becomes  a  mother  cell,  in 
which  case  it  is  said  to  function  as  a  sporangium.  The  wall 
of  a  sporangium  usually  opens,  and  the  spores  are  dis- 
charged, thus  being  free  to  produce  new  plants.  Various 
names  have  been  given  to  asexual  spores  to  indicate  certain 
peculiarities.  As  Algae  are  mostly  surrounded  by  water, 
the  characteristic  asexual  spore  in  the  group  is  one  that 
can  swim  by  means  of  minute  hair-like  processes  or  cilia, 
which  have  the  power  of  lashing  the  water  (Fig.  206,  C). 
These  ciliated  spores  are  known  as  zoospores,  or  "animal- 
like spores,"  referring  to  their  power  of  locomotion ;  some- 
times they  are  called  sivimming  spores,  or  swarm  spores.  It 
must  be  remembered  that  all  of  these  terms  refer  to  the 
same  thing,  a  swimming  asexual  spore. 

This  method  of  reproduction  may  be  indicated  by  a  for- 
mula as  follows  :  P  —  o  —  P  —  o  —  P  —  o  —  P,  which  indi- 
cates that  new  plants  are  not  produced  directly  from  the 
old  ones,  as  in  vegetative  multiplication,  but  that  between 
the  successive  generations  there  is  the  asexual  spore. 

Sexual  spores. — These  cells  are  formed  by  cell  union, 
two  cells  fusing  together  to  form  the  spore.  This  process 
of  forming  a  spore  by  the  fusion  of  two  cells  is  called  the 
sexual  process,  and  the  two  special  cells  (sexual  cells)  thus 
used  are  known  as  gametes  (Fig.  205,  C,  d,  e).  It  must  be 
noticed  that  gametes  are  not  spores,  for  they  are  not  able 
alone  to  produce  a  new  plant ;  it  is  only  after  two  of  them 
have  fused  and  formed  a  new  cell,  the  spore,  that  a  plant 
can  be  produced.  The  spore  thus  formed  does  not  differ 
in  its  power  from  the  asexual  spore,  but  it  differs  very 
much  in  its  method  of  origin. 


THALLOPHYTES:    ALG^  231 

The  gametes  are  organized  within  a  mother  cell,  and  if 
this  cell  is  distinct  from  the  other  cells  of  the  plant  it  is 
called  a  gametajigiitm,  which  means  "gamete  vessel." 

This  method  of  reproduction  may  be  indicated  by  a  for- 
mula as  follows  :  P  =  ^>o  —  P  =  :>o  —  P  =  °>o  —  P, 
which  indicates  that  two  special  cells  (gametes)  are  pro- 
duced by  the  plant,  that  these  two  fuse  to  form  one  (sexual 
spore),  which  then  produces  a  new  plant. 

At  first  the  two  gametes  are  alike  in  size  and  activity, 
and  such  plants  are  said  to  be  isogamous — that  is,  "  with 
similar  gametes."  In  other  plants  the  gametes  become 
very  dissimilar,  one  being  large  and  passive,  and  called  the 
egg;  the  other  being  small  and  active,  and  called  the 
sperm  ;  and  such  plants  are  said  to  be  Jieterogamous — that 
is,  "with  dissimilar  gametes."  The  gametangium  which 
produces  the  egg  is  called  an  oogonium;  that  which  pro- 
duces sperms  is  the  antheridium. 

It  must  not  be  supposed  that  if  a  plant  uses  one  of  these 
three  methods  of  reproduction  (vegetative  multiplication, 
asexual  spores,  sexual  spores)  it  does  not  employ  the  other 
two.  All  three  methods  may  be  employed  by  the  same 
plant,  so  that  new  plants  may  arise  from  it  in  three  differ- 
ent ways. 


16 


CHAPTEE   XVII 

THE  GREAT  GROUPS  OF  ALG^ 

158.  General  characters. — The  Algae  are  distinguished 
among  Thallophytes  by  the  presence  of  chlorophyll.  It 
was  stated  in  a  previous  chapter  that  in  three  of  the  four 
great  groups  another  coloring  matter  is  associated  with  the 
chlorophyll,  and  that  this  fact  is  made  the  basis  of  a  division 
into  Blue-green  Algae  (Cyanophyceae),  Green  Algae  (Chloro- 
phyceae),  Brown  Algae  (Phaeophyceae),  and  Eed  Algae  (Rhodo- 
phyceae).  In  our  limited  space  it  will  be  impossible  to  do 
more  than  mention  a  few  representatives  of  each  group, 
but  they  will  serve  to  illustrate  the  prominent  facts. 

1.  Cyanophyce^  {Blue-gree7i  Algce) 

159.  Gloeocapsa. — These  forms  may  be  found  forming 
blue-green  or  olive-green  patches  on  damp  tree-trunks,  rock, 
walls,  etc.  By  means  of  the  microscope  these  patches  are 
seen  to  be  composed  of  multitudes  of  spherical  cells,  each 
representing  a  complete  Glceocapsa  body.  One  of  the  pecul- 
iarities of  the  body  is  that  the  cell  wall  becomes  mucilagi- 
nous, swells,  and  forms  a  jelly-like  matrix  about  the  work- 
ing cell.  Each  cell  divides  in  the  ordinary  way,  two  new 
Gloeocapsa  individuals  being  formed,  this  method  of  vegeta- 
tive multiplication  being  the  only  form  of  reproduction 
(Fig.  201). 

When  new  cells  are  formed  in   this  way  the   swollen 

mucilaginous  walls  are  apt  to  hold  them  together,  so  that 

presently  a  number  of  cells  or  individuals  are  found  lying 
232 


THE  GREAT   GKOUPS   OF  ALG^ 


23a 


together  imbedded  in  the  jelly-like  matrix  formed  by  the 
wall  material  (Fig.  201).  These  imbedded  groups  of  indi- 
viduals are  spoken  of  as  colonies^  and 
as  colonies  become  large  they  break 
up  into  new  colonies,  the  individual 
cells  composing  them  continuing  to 
divide  and  form  new  individuals. 
This  represents  a  very  simple  life  his- 
tory, in  fact  a  simpler  one  could  hard- 
ly be  imagined. 

160.  Nostoc. — These  forms  occur  in 
jelly-like  masses  in  damp  places.  If 
the  jelly  be  examined  it  will  be  found 
to  contain  imbedded  in  it  numerous 
cells  like  those  of  Gloeocapsa^  but  they 
are  strung  together  to  form  chains  of 
varying  lengths  (Fig.  202).  Th  3  jelly  in 
which  these  chains  are  imbedded  is  the 
same  as  that  found  in  Glmcapsa^  being 
formed  by  the  cell  walls  becoming  mucilaginous  and  swollen. 
One  notable  fact  is  that  all  the  cells  in  the  chain  are  not 

alike,  for  at  irregu- 
lar intervals  there  oc- 
cur larger  colorless 
cells,  an  illustration 
of  the  differentiation 
of  cells.  These  larger 
cells  are  known  as  het- 
erocysts  (Fig.  202,  J), 
which  simply  means 
"other  cells.-'  It  is 
observed  that  when 
the  chain  breaks  up 

Fig.  202.     Nostoc,  a  blue-green  alga,  showing  the  into   fragments    each 

chain-like   filaments*,  and    the    heterocysts  (.1)  fragment  isCOmpOSCd 
which  determine  the  breaking  up  of  the  chain. —  *^ 

Caldwell.  of   the    CClls  bctWCCn 


Fig.  201.  Glcocapsa,  a 
blue-green  alga,  show- 
ing single  cells,  and 
small  groups  which  have 
been  formed  by  division 
and  are  held  together  by 
the  enveloping  muci- 
lage.—Caldwell. 


234 


PLANT  STUDIES 


two  heterocysts.  The  fragments  wriggle  out  of  the  jelly 
matrix  and  start  new  colonies  of  chains,  each  cell  dividing 
to  increase  the  length  of  the  chain.  This  cell  division, 
to  form  new  cells,  is  the  characteristic  method  of  repro- 
duction. 

At  the  approach  of  unfavorable  conditions  certain  cells 
of  the  chain  become  thick- walled  and  well-protected.  These 
cells  which  endure  the  cold  or  other  hardships,  and  upon 
the  return  of  favorable  conditions  produce  new  chains  of 
cells,  are  often  called  spores,  but  they  are  better  called 
"  resting  cells." 

IGl.  Oscillatoria. — These  forms  are  found  as  bluish-green 
slippery  masses  on  wet  rocks,  or  on  damp  soil,  or  freely 
floating.  They  are  simple  filaments,  composed  of  very  short 
flattened  cells  (Fig.  203),  and  the  name 
Oscillatoria  refers  to  the  fact  that  they 
•exhibit  a  peculiar  oscillating  move- 
ment. These  motile  fllaments  are  is- 
olated, not  being  held  together  in  a 
jelly-like  matrix  as  are  the  chains  of 
Nostoc^  but  the  wall  develops  a  cer- 
tain amount  of  mucilage,  which  gives 
the  slippery  feeling  and  sometimes 
forms  a  thin  mucilaginous  sheath 
about  the  row  of  cells. 

The  cells  of  a  filament  are  all  alike, 
except  that  the  terminal  cell  has  its    Fi«-  203.    o^duatona,  9. 

^  blue-green  alga,  showing 

free  surface  rounded,     it  a  filament        a  group  of  filaments  u), 
breaks,   and   a   new   cell   surface   ex-        and  a  single  filament 

,      . ,        ,  T  T     T  more    enlarged    (S).— 

posed,  it   at  once   becomes   rounded.        caldwell. 

If   a    single   cell   of  the    filament    is 

freed  from  all  the  rest,  both  fiattened  ends  become  rounded, 

and  the  cell  becomes  spherical  or  nearly  so.     These  facts 

indicate  at  least  two  important  things  :  (1)  that  the  cell 

wall  is  elastic,  so  that  it  can  be  made  to  change  its  form, 

and  (2)  that  it  is  pressed  upon  from  within,  so  that  if  free 


THE  GREAT  GROUPS  OF  ALG.E  235 

it  will  bulge  outward.  In  all  active  living  cells  there  is 
this  pressure  upon  the  wall  from  within. 

Each  ceil  of  the  Oscillatoria  filament  has  the  power  of 
dividing,  thus  forming  new  cells  and  elongating  the  fila- 
ment. A  filament  may  break  up  into  fragments  of  varying 
lengths,  and  each  fragment  by  cell  division  organizes  a  new 
filament.  Here  again  reproduction  is  by  means  of  vegeta- 
tive multiplication. 

162.  Conclusions. — Taking  Glceocapsa,  Xosfoc,  and  O^cil- 
latoria  as  representatives  of  the  group  Cyanophycese,  or 
"  green  slimes,"  we  may  come  to  some  conclusions  concern- 
ing the  group  in  generaL  The  plant  body  is  very  simple, 
consisting  of  single  cells,  or  chains  and  filaments  of  cells. 
Although  in  Nostoc  and  Oscillatoria  the  cells  are  organized 
into  chains  and  filaments,  each  cell  seems  to  be  able  to  live 
and  act  independently,  and  the  chain  and  filament  seem  to 
be  little  more  than  colonies  of  individual  cells.  In  this 
sense,  all  of  these  plants  may  be  regarded  as  one-celled. 

Differentiation  is  exhibited  in  the  appearance  of  hetero- 
cysts  in  Nostoc^  peculiar  cells  which  seem  to  be  connected 
in  some  way  with  the  breaking  up  of  filamentous  colonies, 
although  the  Oscillatoria  filament  breaks  up  without  them. 

The  power  of  motion  is  also  well  exhibited  by  the  group, 
the  free  filaments  of  Oscillatoi^ia  moving  almost  continually, 
and  the  imbedded  chains  of  Nostoc  at  times  moving  to  es- 
cape from  the  restraining  mucilage. 

The  whole  group  also  shows  a  strong  tendency  in  the 
cell-wall  material  to  become  converted  into  mucilage  and 
much  swollen,  a  tendency  which  reaches  an  extreme  expres- 
sion in  such  forms  as  Nostoc  and  Glmocapsa. 

Another  distinguishing  mark  is  that  reproduction  is 
exclusively  by  means  of  vegetative  multiplication,  through 
ordinary  cell  division  or  fission,  which  takes  place  very 
freely.  Individual  cells  are  organized  with  heavy  resistant 
walls  to  enable  them  to  endure  the  winter  or  other  unfavor- 
able conditions,  and  to  start  a  new  series  of  individuals 


236 


PLANT   STUDIES 


upon  the  return  of  favorable  conditions.  These  may  be 
regarded  as  resting  cells.  So  notable  is  the  fact  of  repro- 
duction by  fission  that  Cyanophyceae  are  often  separated 
from  the  other  groups  of  Algae  and  spoken  of  as  "  Fission 
Algae,"  which  put  in  technical  form  becomes  Schizophyceae. 
In  this  particular,  and  in  several  others  mentioned  above, 
they  resemble  the  "  Fission  Fungi  "  (Schizomycetes),  com- 
monly called  "bacteria,"  so  closely  that  they  are  often 
associated  with  them  in  a  common  group  called  "Fis- 
sion plants "  (Schizophytes),  distinct  from  the  ordinary 
Algae  and  Fungi. 


2.  Chlorophtce^  {Green  Algce). 

163.  Pleurococcus. — This  may  be  taken  as  a  type  of  one- 
celled  Green  Alga?.  It  is  most  commonly  found  in  masses 
covering  damp  tree-trunks,  etc.,  and  looking  like  a  green 
stain.  These  fine- 
ly granular  green 
masses  are  found 
to  be  made  up 
of  multitudes  of 
spherical  cells  re- 
sembling those  of 
Glceocapsa^  except 
that  there  is  no 
blue  with  the  chlo- 
rophyll, and  the 
cells  are  not  im- 
bedded in  such 
jelly-like  masses. 
The  cells  may  be 
solitary,  or  may 
cling  together  in 

colonies  of  various  sizes  (Fig.  204).     Like  Oloeocapsa^  a  cell 
divides  and  forms  two  new  cells,  the  only  reproduction 


Fig.  204.  Pleurococcus,  a  one-celled  green  alga  :  A,  show- 
ing the  adult  form  with  its  nucleus  ;  B,  C,  D,  E, 
various  stages  of  division  (fission)  in  producing  new 
cells  ;  F,  colonies  of  cells  which  have  remained  in 
contact.— C  aldwell. 


THE  GREAT   GROUPS   OF   ALGM 


237 


being  of  this  simple  kind.  It  is  evident,  therefore,  that  the 
group  Chlorophyceae  begins  with  forms  just  as  simple  as 
are  to  be  found  among  the  Cyanophyceae. 

Pleurococcus  is  used  to  represent  the  group  of  Protococ- 
cus  forms,  one-celled  forms  which  constitute  one  of  the 
subdivisions  of  the  Green  AlgaB.  It  should  be  said  that 
Pleurococcus  is  possibly  not  a  Protococcus  form,  but  may 
be  a  reduced  member  of  some  higher  group ;  but  it  is  so 
common,  and  represents  so  well  a  typical  one-celled  green 
alga,  that  it  is  used  in  this  connection.  It  should  be 
known,  also,  that  while  the  simplest  Protococcus  forms  re- 
produce only  by  fission,  others  add  to  this  the  other  meth- 
ods of  reproduction. 

164.  Ulothrix. — This  form  is  very  common  in  fresh  wa- 
ters, being  recognized  easily  by  its  simple  filaments  com- 
posed of  short  squarish  cells,  each  cell  containing  a  single 
conspicuous  cylindrical  chloroplast  (Fig.  205). 

The  cells  are  all  alike,  excepting  that  the  lowest  one  of 
the  filament  is  mostly  colorless,  and  is  elongated  and  more 
or  less  modified  to  act  as  a  holdfast,  anchoring  the  filament 
to  some  firm  support.  With  this  exception  the  cells  are  all 
nutritive  ;  but  any  one  of  them  has  the  power  of  organizing 
for  reproduction.  This  indicates  that  at  first  nutritive  and 
reproductive  cells  are  not  distinctly  diiferentiated,  but  that 
the  same  cell  may  be  nutritive  at  one  time  and  rei^roductive 
at  another.  This  plant  uses  cell  division  to  multiply  the 
cells  of  a  filanient,  and  to  develop  new  filaments  from  frag- 
ments of  old  ones ;  but  it  also  produces  asexual  spores  in 
the  form  of  zoospores,  and  gametes  which  conjugate  and 
form  zygotes.  Both  zoospores  and  zygotes  have  the  power 
of  germination — that  is,  the  power  to  begin  the  develop- 
ment of  a  new  plant.  In  the  germination  of  tlie  zygote 
a  new  filament  is  not  produced  directly,  but  there  are 
formed  within  it  zoospores,  each  of  which  produces  a 
new  filament  (Fig.  205,  Fy  G).  All  three  kinds  of  repro- 
duction are  represented,  therefore,  but  the  sexual  method 


238 


PLANT  STUDIES 


is  the  low  type  called  isogamy,  the  pairing  gametes  being 
alike. 

Ulothrix  is  taken  as  a  representative  of  the  Conferva 
forms,  the  most  characteristic  group  of  Chlorophyceae.     All 


Pig.  205.  Ulothrix.  a  Conferva  form.  A,  base  of  filament,  showing  lowest  holdfast 
cell  and  five  vegetative  cells,  each  with  its  single  conspicuous  cylindrical  chloro- 
plast  (seen  in  section)  inclosing  a  nucleus  ;  B,  four  cells  containing  numerous 
small  zoospores,  the  others  emptied;  C,  fragment  of  a  filament  showing  one  cell 
(a)  containing  four  zoospores,  another  zoospore  (b)  displaying  four  cilia  at  its 
pointed  end  and  just  having  escaped  from  its  cell,  another  cell  (c)  from  which 
most  of  the  small  biciliate  gametes  have  escaped,  gametes  pairing  {d),  and  the 
resulting  zygotes  {e) ;  D,  beginning  of  new  filament  from  zoospore  ;  E,  feeble 
filaments  formed  by  the  small  zoospores ;  F,  zygote  growing  after  rest ;  G, 
zoospores  produced  by  zygote.— Caldwell,  except  F  and  G,  which  are  after 

DODEL-PORT. 


the  Conferva  forms,  however,  are  not  isogamous,  as  will  be 
illustrated  by  the  next  example. 

165.  (Edogonium. — This  is  a  very  common  green  alga, 
found  in  fresh  waters  (Fig.  206).  The  filaments  are  long  and 
simple,  the  lowest  cell  acting  as  a  holdfast,  as  in  Ulothrix 


Fig.  200.  CEdogonium  nodosimi,  a  Conferva  form  :  .4,  portion  of  a  filament  showing  a 
vegetative  cell  with  its  nucleus  (d),  an  oogonium  (a)  filled  bj'  an  e^g  packed  with 
food  material,  a  second  oogonium  {c)  containing  a  fertilized  egg  or  oospore  as 
Bhown  by  the  heavy  wall,  and  two  antheridia  (6),  each  containing  two  sperms;  B, 
another  filament  showing  antheridia  (a)  from  which  two  sperms  (6)  have  escaped, 
a  vegetative  ceil  with  its  nucleus,  and  an  oogonium  which  a  sperm  (c)  has  entered 
and  is  coming  in  contact  with  the  egg  whose  nucleus  ((/)  may  be  seen;  C,  a  zoo- 
spore which  has  been  formed  in  a  vegetative  cell,  showing  the  crown  of  cilia  and 
the  clear  apex,  as  in  the  sjierms;  T).  a  zoospore  producing  a  new  filament,  putting 
out  a  holdfast  at  base  and  elongating:  E,  a  further  stage  of  development;  F.  the 
four  zoospores  formed  by  the  oosjiore  when  it  germinates.— Caldwell,  except 
Cand  F,  which  are  after  Pringsueim. 


240 


PLANT   STUDIES 


(§  164).  The  other  cells  are  longer  than  in  Ulothrix,  eaoh 
cell  containing  a  single  nucleus  and  apparently  several 
chloroplasts,  but  really  there  is  but  one  large  complex 
chloroplast. 

The  cells  of  the  filament  have  the  power  of  division,  thus 
increasing  the  length  of  the  filament.  Any  cell  also  may 
act  as  a  sporangium,  the  contents  of  a  mother  cell  organiz- 
ing a  single  large  asexual  spore,  which  is  a  zoospore.  The 
zoospore  escapes  from  the  mother  cell  into  the  water,  and  at 
its  more  pointed  clear  end  there  is  a  little  crown  of  cilia,  by 
means  of  which  it  swims  about  rapidly  (Fig.  206,  C).  After 
moving  about  for  a  time  the  zoospore  comes  to  rest,  attaches 
itself  by  its  clear  end  to  some  support,  elongates,  begins  to 
divide,  and  develops  a  new  filament  (Fig.  206,  D,  E). 

Other  cells  of  the  filament  become  very  different  from 
the  ordinary  cells,  swelling  out  into  globular  form  (Fig. 
206,  A,  B),  and  each  such  cell  organizes  within  itself  a 
single  large  egg  (oosphere).  As  the  egg  is  a  female  gamete, 
the  large  globular  cell  which  produces  it,  and  which  is  dif- 
ferentiated from  the  other  cells  of  the  body,  is  the  oogo- 
nium. A  perforation  in  the  oogonium  wall  is  formed  for 
the  entrance  of  sperms. 

Other  cells  in  the  same  filament,  or  in  some  other  fila- 
ment, are  observed  to  differ  from  the  ordinary  cells  in 
being  much  shorter,  as  though  an  ordinary  cell  had  been 
divided  several  times  without  subsequent  elongation  (Fig. 
206,  A,f,  B,  a).  In  each  of  these  short  cells  one  or  two 
sperms  are  organized,  and  therefore  each  short  cell  is  an 
antheridium.  When  the  sperms  are  set  free  they  are  seen 
to  resemble  very  small  zoospores,  having  the  same  little 
crown  of  cilia  at  one  end. 

The  sperms  swim  actively  about  in  the  vicinity  of  the 
oogonia,  and  sooner  or  later  one  enters  the  oogonium 
through  the  perforation  provided  in  the  wall,  and  fuses 
with  the  egg  (Fig.  206,  B,  c).  As  a  result  of  this  act  of  fer- 
tilization an  oospore  is  formed,  which  organizes  a  firm  wall 


THE   GEEAT   GROUPS   OF  ALG^ 


241 


about  itself.  This  firm  wall  indicates  that  the  oospore  is 
not  to  germinate  immediately,  but  is  to  pass  into  a  resting 
condition.  Spores  which  form  heavy  walls  and  pass  into 
the  resting  con- 
dition are  often 
spoken  of  as  "  rest- 
ing spores,"  and  it 
is  very  common 
for  the  zygotes 
and  oospores  to 
be  resting  spores. 
These  resting 
spores  enable  the 
plant  to  endure 
through  unfavor- 
able conditions, 
such  as  failure  of 
food  supply,  cold, 
drought,  etc. 
When  favorable 
conditions  return, 
the  protected  rest- 
ing spore  is  ready 
for  germination. 

When  the 
oospore  of  CEdogo- 
nium  germinates 
it  does  not  develop  directly  into  a  new  filament,  but  the 
contents  become  organized  into  four  zoospores  (Fig.  206,  F)^ 
which  escape,  and  each  zoospore  develops  a  filament.  In 
this  way  each  oospore  may  give  rise  to  four  filaments. 

It  is  evident  that  (Eclogonium  is  a  heterogamous  plant, 
and  is  another  one  of  the  Conferva  forms.  Conferva  bodies 
are  not  always  simple  filaments,  as  are  those  of  llothrix 
and  (Edogonium^hvii  they  are  sometimes  extensively  branch- 
ing filaments,  as  in  Cladophora^  a  green  alga  very  common 


Fig.  207.  Cladophora,  a  branching  green  alga,  a  very- 
small  part  of  the  plant  being  shown.  The  branches 
arise  at  the  upper  ends  of  cells,  and  the  cells  are 
ccenocytic— Caldwell. 


242 


PLANT   STUDIES 


in  rivers  and  lakes  (Fig.  207).  The  cells  are  long  and 
densely  crowded  with  chloroplasts ;  and  in  certain  cells  at 
the  tips  of  branches  large  numbers  of  zoospores  are  formed, 
which  have  two  cilia  at  the  pointed  end,  and  hence  are  said 
to  be  hiciliate. 

166.  Vaucheria. — This  is  one  of  the  most  common  of  the 
Green  Algae,  found  in  felt-like  masses  of  coarse  filaments  in 
shallow  water  and  on  muddy  banks,  and  often  called  "  green 


Fig.  208.  Vancheria  geminata,  a  Siphon  form,  showing  a  portion  of  the  ccenocytic 
body  {A)  which  has  sent  out  a  branch  at  the  tip  of  which  a  sporangium  {B) 
formed,  within  which  a  large  zoospore  was  organized,  and  from  which  (Z>)  it  is 
discharged  later  as  a  large  multiciliate  body  ( C),  which  then  begins  the  develop- 
ment of  a  new  ccenocytic  body  (^E ).— Caldwell. 


felt."  The  filament  is  very  long,  and  usually  branches  ex- 
tensively, but  its  great  peculiarity  is  that  there  is  no  parti- 
tion wall  in  the  whole  body,  which  forms  one  long  continuous 
cavity  (Fig.  208).  This  is  sometimes  spoken  of  as  a  one- 
celled  body,  but  it  is  a  mistake.  Imbedded  in  the  exten- 
sive cytoplasm  mass,  which  fills  the  whole  cavity,  there  are 
not  only  very  numerous  chloroplasts,  but  also  numerous 
nuclei.     As  has  been  said,  a  single  nucleus  with  some  cyto- 


THE  GREAT  GROUPS  OE  ALG^  243 

plasm  organized  about  it  is  a  cell,  whether  it  has  a  wall  or 
not.  Therefore  the  body  of  Vaucheria  is  made  up  of  as 
many  cells  as  there  are  nuclei,  cells  whose  protoplasmic 
structures  have  not  been  kept  separate  by  cell  walls.  Such 
a  body,  made  up  of  numerous  cells,  but  with  no  partitions, 
is  called  a  coenocyte^  or  it  is  said  to  be  cmnocytic.  Vaucheria 
represents  a  great  group  of  Chlorophyceae  whose  members 
have  coenocytic  bodies,  and  on  this  account  they  are  called 
the  Siphon  forms. 

Vaucheria  produces  very  large  zoospores.  The  tip  of  a 
branch  becomes  separated  from  the  rest  of  the  body  by  a 
partition  and  thus  acts  as  a  sporangium  (Fig.  208,  B).  In 
this  improvised  sporangium  the  whole  of  the  contents  or- 
ganize a  single  large  zoospore,  which  is  ciliated  all  over, 
escapes  by  squeezing  through  a  perforation  in  the  wall 
(Fig.  208,  C'),  swims  about  for  a  time,  and  finally 
develops  another  Vaucheria  body  (Figs.  208,  E, 
209).  It  should  be  said  that  this  large  body, 
called  a  zoospore  and  acting  like  one,  is  really 
a  mass  of  small  biciliate  zoospores,  just  as  the 


Fig.  209.  A  yonng  Vancheria  frerminatinc:  from  a 
spore  isp),  and  showing  the  holdfast  {w).~ 
After  Sachs. 

apparently  one-celled  vegetative  body  is  really  composed  of 
many  cells.  In  this  large  compound  zoospore  there  are 
many  nuclei,  and  in  connection  with  each  nucleus  two  cilia 
are  developed.  Each  nucleus  witli  its  cytoplasm  and  two 
cilia  represents  a  small  biciliate  zoospore,  such  as  those  of 
Cladophora,  §165. 

Antheridia  and  oogonia  are  also  developed.  In  a  com- 
mon form  tliese  two  sex  organs  appear  as  short  special 
branches  developed  on  tlie  side  of  the  large  coenocytic  body. 


244 


PLANT   STUDIES 


and  cut  off  from  the  general  cavity  by  partition  walls  (Fig. 
210).    The  oogonium  becomes  a  globular  cell,  which  usually 


Fig.  210.  Vaucheria  sessilis,  a  Siphon  form, 
showing  a  portion  of  the  ccenocytic  body,  an 
antheridial  branch  (.4)  with  an  empty  anthe- 
ridium  {a)  at  its  tip ;  and  an  oogonium  (B) 
containing  an  oospore  (c)  and  showing  the 
opening  (/)  through  which  the  sperms  passed 
to  reach  the  egg.— Caldwell, 


develops  a  perforated  beak  for 
the  entrance  of  the  sperms,  and 
organizes  within  itself  a  single 
large  Qgg  (Fig.  210,  B).  The  an- 
theridium  is  a  much  smaller  cell, 
within  which  numerous  very  small 
sperms  are  formed  (Fig.  210,  .1,  a). 
The  sperms  are  discharged,  swarm 
about  the  oogonium,  and  finally 
one  passes  through  the  beak  and 
fuses  with  the  Qgg^  the  result  be- 
ing an  oospore.  The  oospore  or- 
ganizes a  thick  wall  and  becomes 
a  resting  spore. 

It  is  evident  that  Vaucheria  is  heterogamous,  but  all 
the  other  Siphon  forms  are  isogamous,  of  which  BotrycUum 
may  be  taken  as  an  illustration  (Fig.  211). 

167.  Spirogyra. — This  is  one  of  the  commonest  of  the 
"pond  scums,"  occurring  in  slippery  and  often  frothy 
masses  of  delicate  filaments  floating  in  still  water  or  about 


Fig.  211.  Botrydhim,  one  of 
the  Siphon  forms  of  green 
algae,  the  whole  body  con- 
taining one  continuous  cav- 
ity, with  a  bulbous,  chloro- 
phyll-containing portion, 
and  root -like  branches 
which  penetrate  the  mud 
in  which  the  plant  grows. 
—Caldwell. 


THE  GREAT  GROUPS  OF  ALGiE 


245 


springs.     The  filaments  are  simple,  and  are  not  anchored  by 
a  special  basal  cell,  as  in  Ulothrix  and  (Edogonium.     The 


Fig.  212.  Spirogyra,  a  Conjugate  form,  showing  one  complete  cell  and  portions  of 
two  others.  The  band-like  chloroplasts  extend  in  a  spiral  from  one  end  of  the 
cell  to  the  other,  in  them  are  imbedded  nodule-like  bodies  {pyrenoids),  and  near 
the  center  of  the  cell  the  nucleus  is  swung  by  radiating  strands  of  cytoplasm.— 
Caldwell. 

cells  contain  remarkable  chloroplasts,  which  are  bands  pass- 
ing spirally  about  within  the  cell  wall.     These  bands  may 


Fig.  213.  Spirogyra,  showing  conjugation  :  A,  conjugating  tubes  approaching  each 
other;  B,  tubes  in  contact  but  end  walls  not  absorbed:  C,  tube  complete  and  con- 
tents of  one  cell  passing  through;  D,  a  completed  zygospore.— Caldv^-bll. 


246 


PLANT  STDDIES 


be  solitary  or  several  in  a  cell,  and  form  very  striking  and 
conspicuous  objects  (Figs.  212,  213). 

Spirogyra  and  its  associates  are  further  peculiar  in  pro- 
ducing no  asexual  spores,  and  also  in  the  method  of  sexual 
reproduction.  Two  adjacent  filaments  put  out  tubular 
processes  toward  one  another.  A  cell  of  one  filament  sends 
out  a  process  which  seeks  to  meet  a  corresponding  process 
from  a  cell  of  the  other  filament.  When  the  tips  of  two 
such  processes  come   together,  the   end  walls   disappear, 


Fig.  214.  Spirogyra,  showing  some  common  exceptions.  At  A  two  cells  have  been 
connected  by  a  tube,  but  without  fusion  a  zygote  has  been  organized  in  each  cell; 
also,  the  upper  cell  to  the  left  has  attempted  to  conjugate  with  the  cell  to  the 
right.  At  B  there  are  cells  from  three  filaments,  the  cells  of  the  central  one  hav- 
ing conjugated  with  both  of  the  others.— Caldwell, 


and  a  continuous  tube  extending  between  the  two  cells  is 
organized  (Figs.  213, 214).  When  many  of  the  cells  of  two 
parallel  filaments  become  thus  united,  the  appearance  is 
that  of  a  ladder,  with  the  filaments  as  the  side  pieces,  and 
the  connecting  tubes  as  the  rounds. 

While  the  connecting  tube  is  being  developed  the  con- 
tents of  the  two  cells  are  organizing,  and  after  the  comple- 
tion of  the  tube  the  contents  of  one  cell  pass  through  and 
enter  the  other  cell,  fuse  with  its  contents,  and  a  sexual 


THE   GREAT   GROUPS   OF  ALG^ 


247 


spore  is  organized.  As  the  gametes 
look  alike,  the  process  is  conjuga- 
tion, and  the  sex  spore  is  a  zygote, 
which,  with  its  heavy  wall,  is  rec- 
ognized to  be  a  resting  spore.  At 
the  beginning  of  each  growing 
season,  the  well-protected  zygotes 
which  have  endured  the  winter 
germinate  directly  into  new  Spi- 
rogyra  filaments. 

On  account  of  this  peculiar 
style  of  sexual  reproduction,  in 
which  gametes  are  not  discharged, 
but  reach  each  other  through  spe- 
cial tubes,  S2)irogyra  and  its  allies 
are  called  Conjugate  forms — that 
is,  forms  whose  bodies  are  "  yoked 
together  "  during  the  fusion  of  the 
gametes. 

In  some  of  the  Conjugate  forms 
the  zygote  is  formed  in  the  connect- 
ing tube  (Fig.  215,  A)^  and  some- 
times zygotes  are  formed  without 
conjugation  (Fig.  315,  B).  Among 
the  Conjugate  forms  the  Desmids 
are  of  great  interest  and  beauty, 
being  one-celled,  the  cells  being 
organized  into  two  distinct  halves 
(Fig.  216). 

168.  Conclusions.  —  The  Green 
Algae,  as  indicated  by  the  illustra- 
tions given  above,  include  simple 
one-celled  forms  which  reproduce 
by  fission,  but  they  are  chiefly  fila- 
mentous forms,  simple  or  brandling.  These  filamentous 
bodies  either  have  the  cells  separated  from  one  another 
17 


Fig.  215.  Two  Conjugate  forms  : 
A  {Mougeotia),  showing  for- 
mation of  zygote  in  conjuga- 
ting tube  ;  B,  C  (Gonatone- 
ma),  (Showing  formation  of 
zygote  without  conjugation. 
— After  WiTTRocK. 


248 


PLAKT   tsTUDIES 


by  walls,  or  they  are  coenocytic,  as  in  the  Siphon  forms. 
The  characteristic  asexual  spores  are  zoospores,  but  these 
may  be  wanting,  as  in  the  Conjugate  forms.  In  addition 
to  asexual  reproduction,  both  isogamy  and  heterogamy  are 
developed,  and  both  zygotes  and  oospores  are  resting  spores. 


Fig.  216.   A  group  of  Desmids,  one-celled  Conjugate  forms,  showing  various  pat- 
terns, and  the  cells  organized  into  distinct  halves. — After  Kerner. 

The  Green  Algae  are  of  special  interest  in  connection 
with  the  evolution  of  higher  plants,  which  are  supposed  by 
some  to  have  been  derived  from  them. 


3.  Ph^ophyce^  (Brown  Algce) 

169.  General  characters.— The  Blue-green  Algae  and  the 
Green  Algae  are  characteristic  of  fresh  water,  but  the  Brown 
Algae,  or  "  kelps,"  are  almost  all  marine,  being  very  charac- 


THE   GREAT   GROUPS   OF   ALG^ 


249 


teristic  coast  forms.  All  of  them  are  anchored  by  holdfasts, 
which  are  sometimes  highly  developed  root-like  structures ; 
and  the  yellow,  brown,  or  olive-green  floating 
bodies  are  buoyed  in  the  water  usually  by  the 
aid  of  floats  or  air-bladders,  which  are  often 
very  conspicuous.  The  kelps  are  most  highly 
developed  in  the  colder  waters,  and  form  much 
of  the  "wrack,"  "tangle,"  etc.,  of  the  coasts. 
The  group  is  well  adapted  to 
live  exposed  to  waves  and  cur- 
rents with  its  strong  holdfasts, 
air-bladders,  and  tough  leathery 
bodies.  Certain  Brown  Algae,  as 
Ectocarpus  (Fig.  18),  are  of 
great  interest  on  account  of 
their  possible  relation  to  the 
evolution  of  higher  plants.  It  is 
in  this  group  that  we 
have  found  our  only 
suggestions  as  to  the 
origin  of  the  complex 
sex-organs  occurring 
in  Bryophytes  and 
Pteridophytes. 

170.  The  plant 
body. — There  is  very 
great  diversity  in  the 
structure  of  the 
plant  body.  Some 
of  them,  as  Ectocar- 
pus (Fig.  217),  are  fil- 
amentous forms,  like 
the  Confervas  among 
the  Green  Algae,  but 
others  are  very  much  more  complex.  The  thallus  of  Lam- 
inaria  is  like  a  huge  floating  leaf,  frequently  nine  to  ten 


Fig.  217.  A  brown  alga  { Ectocarpus).  showing  a 
body  consisting  of  a  simple  filament  which  puts 
out  branches  (A),  some  sporangia  (2?)  contain- 
ing zoospores,  and  gametangia  {€')  containing 
gametes.— Caldwell. 


S'^-'^ 


^^ 


U-ii.i!. 


«»^ 


^^-r 


Fig.  218.    A  group  of  brown  seaweeds  {Laminarias).    Note  the  various  habits  of 
the  plant  body  with  its  leaf -like  thallus  and  root- like  holdfasts.— After  Kerner. 


THE   GKEAT   GROUPS   OF   ALG^ 


251 


feet  long,  whose  stalk  develops  root-like  holdfasts  (Fig.  218). 
The  largest  body  is  developed  by  an  Antarctic  Laminaria 
form,  which  rises  to  the  surface  from  a  sloping  bottom  with 
a  floating  thallus  six  hundred  to  nine  hundred  feet  long. 
Other  forms  rise  from  the  sea  bottom  like  trees,  with 
thick  trunks,  numerous  branches,  and  leaf-like  appendages^ 

The  common  Fucus^ 
or  "  rock  weed,"  is  rib- 
bon-form and  constantly 
branches  by  forking  at 
the  tip  (Fig.  219).  This 
method  of  branching  is 
called  dicliotomous^  as  dis- 
tinct from  that  in  which 
branches  are  put  out 
from  the  sides  of  the  axis 
{monopodial).  The  swol- 
len air-bladders  distrib- 
uted throughout  the  body 
are  very  conspicuous. 

The  most  differenti- 
ated thallus  is  that  of 
Sargassum  (Fig.  220),  or 
"  gulf  weed,"  in  which 
there  are  slender  branch- 
ing stem-like  axes  bearing 
lateral  members  of  various 
kinds,  some  of  them  like 
ordinary  foliage  leaves ; 
others  are  floats  or  air- 
bladders,  which  sometimes 

resemble  clusters  of  berries;  and  other  branches  bear  the 
sex  organs.  All  of  these  structures  are  but  different  regions 
of  a  branching  thallus.  Sargassum  forms  are  often  torn 
from  their  anchorage  by  the  waves  and  carried  away  from 
the  coast  by  currents,  collecting  in  the  great  sea  eddies 


Fig.  219.  Fragment  of  a  common  brown 
alga  (Fvcus),  showing  the  body  with 
dichotomoiis  branching  and  bladder-like 
air-bladders.— After  Luerssen. 


252 


PLANT   STUDIES 


produced  by  oceanic  currents  and  forming  the  so-called 
*'  Sargasso  seas,"  as  that  of  the  North  Atlantic. 


Fig.  220.  A  portion  of  a  brown  alga  {Sargassum),  showing  the  thallus  differentiated 
into  stem-like  and  leaf-like  portions,  and  also  the  bladder-like  floats.— After  Ben- 
nett and  Murray. 


171.  Reproduction. — The  two  main  groups  of  Brown 
Algae  differ  from  each  other  in  their  reproduction.  One, 
represented  by  the  Laminarias  and  a  majority  of  the  forms, 
produces  zoospores  and  is  isogamous  (Fig.  217).  The  zoo- 
spores and  gametes  are  peculiar  in  having  the  two  cilia 
attached  at  one  side  rather  than  at  an  end ;  and  they  re- 
semble each  other  very  closely,  except  that  the  gametes 
fuse  in  pairs  and  form  zygotes. 


-    ^i-' 


ZJrtlZT     "^  °'  i^««...  Showing  the  eight  eggs  (six  in  sight)  dis- 

rom   hV^eltneTr:^^^^^^^^  by  a  membrane  U),  eggs  liberated 

orally  bfcrareeZmfV^anT'"'  ^^"^"'ning  sperms  (C).  the  discharged  lat- 

After  Singer  ^^'    ^  '^^'  surrounded  by  swarming  sperms  ,F,  II).- 


254 


PLANT   STUDIES 


The  other  group,  represented  by  Fucus  (Fig.  221),  pro- 
duces no  asexual  spores,  but  is  heterogamous.  A  single 
oogonium  usually  forms  eight  eggs  (Fig.  221,  A)^  which  are 
discharged  and  float  freely  in  the  water  (Fig.  221,  E).  The 
antheridia  (Fig.  221,  C)  produce  numerous  minute  laterally 
biciliate  sperms,  which  are  discharged  (Fig.  221,  G)^  swim 
in  great  numbers  about  the  large  eggs  (Fig.  221,  F,  H), 
and  finally  one  fuses  with  an  egg^  and  an  oospore  is  formed. 
As  the  sperms  swarm  very  actively  about  the  egg  and  im- 
pinge against  it  they  often  set  it  rotating.  Both  antheridia 
and  oogonia  are  formed  in  cavities  of  the  thallus. 


4.   Ehodophyce^  {Red  A\ 

172.  General  characters. — On  account  of  their  red  colora- 
tion these  forms  are  often  called  Floridem.    They  are  mostly 

marine  forms,  and  are 
anchored  by  holdfasts 
of  various  kinds.  They 
belong  to  the  deepest 
waters  in  which  Algae 
grow,  and  it  is  probable 
that  the  red  coloring 
matter  which  character- 
izes them  is  associated 
with  the  depth  at  which 
they  live.  The  Red 
Algae  are  also  a  high- 
ly specialized  line,  and 
will  be  mentioned  very 
briefly. 

173.  The  plant  body. 
—  The  Eed  Algae,  in 
general,  are  more  deli- 
cate than  the  Brown 
Algae,  or  kelps,  their  graceful  forms,  delicate  texture,  and 
Mghtly  tinted  bodies  (shades  of  red,  violet,  dark  purple. 


«s&^# 


Fig.  222.  A  red  alga  (Gigariina),  showing 
branching  habit,  and  "fruit  bodies."— 
After  ScHENCK. 


Fig.  224. 


A  red  alga  {Dasya),  showing  a  finely  divided  thallus  body. 
Caldwell. 


Fig  225.    A  red  alga  {Rabdonia),  ehowing  holdfasts  and  branching  thalhis  body.— 

Caldwell. 


Fig.  226.    A  red  alga  {Ptilota),  whose  branching  body  resembles  moss.- 
Caldwell. 


THE   GREAT   GROUPS   OF   ALG^E 


259 


and  reddish-brown)  making  them  very  attractive.  They 
show  the  greatest  variety  of  forms,  branching  filaments, 
ribbons,  and  filmy  plates  prevailing,  sometimes  branching 
very  profusely  and  delicately,  and  resembling  mosses  of 
fine  texture  (Figs.  222,  223,  224,  225,  226).  The  differen- 
tiation of  the  thallus  into  root  and  stem  and  leaf-like  struc- 
tures is  also  common,  as  in  the  Brown  Algse. 

174.  Reproduction. — Eed  Algae  are  very  peculiar  in  both 
their  asexual  and  sexual  reproduction.  A  sporangium  pro- 
duces just  four  asexual  spores,  but  they  have  no  cilia  and 
no  power  of  motion.  They 
can  not  be  called  zoospores, 
therefore,  and    as    each    spo- 


FiG.  227.  A  red  alga  ( Callitkamnion).  show- 
ing sporangium  (.4),  and  the  tetraspores 
discharged  (^).— After  Thuret. 


Fig.  228.  A  red  alga  (XefnaUon) :  A, 
sexual  branches,  showing  antheri- 
dia  (a),  oogonium  (o)  with  its  trich- 
ogyne  (0,  to  which  are  attached  two 
spermatia  (s) ;  B,  beginning  of  a 
cystocarp  (o),  the  trichogyne  (t)  still 
showing  ;  C.  an  almost  mature  cys- 
tocarj)  (o\  with  the  disorganizing 
trichogyne  (0-— After  Kny. 


rangium  always  produces  just 
four,  they  have  been  called 
tetrasj)ores  (Fig.  227). 

Eed  Algge  are  also  heterog- 
amous,  but  the  sexual  process  has  been  so  much  and  so 
variously  modified  that  it  is  very  poorly  understood.  The 
antheridia  (Fig.  228,  A,  a)  develop  sperms  which,  like  the 
tetraspores,  have  no  cilia  and  no  power  of  motion.     To  dis- 


260 


PLANT   STUDIES 


tinguish  them  from  the  ciliated  sperms,  or  spermatozoids, 
which  have  the  power  of  locomotion,  these  motionless  male 
gametes  of  the  Bed  Algae  are  usually  called  spermatid 
(singular,  spermatium)  (Fig.  228,  .4,  s). 

The  oogonium  is  very  pe- 
culiar, being  differentiated 
into  two  regions,  a  bulbous 
base  and  a  hair-like  process 
{tricliogyne)^  the  whole  struc- 
ture resembling  a  flask  with  a 
long,  narrow  neck,  excepting 
that  it  is  closed  (Fig.  228, 
J,  0,  t).  Within  the  bulbous 
part  fertilization  usually  takes 
place  ;  a  spermatium  attaches 
itself  to  the  trichogyne  (Fig. 
228,  A^  s) ;  at  the  point  of 
contact  the  two  walls  become 
perforated,  and  the  contents 
of  the  spermatium  thus  enter 
the  trichogyne,  and  so  reach 
the  bulbous  base  of  the  oogo- 
nium. The  above  account 
represents  the  very  simplest 
conditions  of  the  process  of 
fertilization  in  this  group,  and 
gives  no  idea  of  the  great  and 
puzzling  complexity  exhibited 
by  the  majority  of  forms. 

After  fertilization  the  trich- 
ogyne wilts,  and  the  bulbous 
base  in  one  way  or  another  de- 
velops a  conspicuous  structure 
called  the  cystocarp  (Figs.  228,  229),  which  is  a  case  con- 
taining asexual  spores ;  in  other  words,  a  spore  case,  or  kind 
of  sporangium.     In  the  life  history  of  a  red  alga,  there- 


PiG.  229.  A  branch  of  Polysiphonia, 
one  of  the  red  algae,  showing  the 
lows  of  cells  composing  the  body 
{A),  small  branches  or  hairs  {B), 
and  a  cystocarp  (C)  with  escaping 
spores  (Z>)  which  have  no  cilia  (car- 
pospores). — Caldw^ell. 


THE  GREAT  GROUPS  OF  ALGiE 


261 


fore,  two  sorts  of  asexual  spores  are  produced :  (1)  the 
tetrasijores^  developed  in  ordinary  sporangia;  and  (2)  the 
carpo8pores^  developed  in  the  cystocarp,  which  has  been 
produced  as  the  result  of  fertilization. 

OTHER  CHLOROPHYLL-CONTAINIKG  THALLOPHYTES 

175.  Diatoms. — These  are  peculiar  one-celled  forms,  which 
occur  in  very  great  abundance  in  fresh  and  salt  waters. 


FiQ.  230.  A  group  of  Diatoms  :  c  and  c?,  top  and  side  views  of  the  same  form;  e,  colony 
of  stalked  forms  attached  to  an  alga; /and  g,  top  and  side  views  of  the  form  shown 
at  e\  h,  sl  colony;  i,  a  colony,  the  top  and  side  view  shown  at  ^•.— After  Kerner. 


They  are  either  free-swimming  or  attached  by  gelatinous 
stalks;  solitary,  or  connected  in  bands  or  chains,  or  im- 
bedded in  gelatinous  tubes  or  masses.  In  form  they  are 
rod-shaped,  boat-shaped,  elliptical,  wedge-shaped,  straight 
or  curved  (Fig.  230). 


262 


PLANT   STUDIES 


The  chief  peculiarity  is  that  the  wall  is  composed  of  two 
valves,  one  of  which  fits  into  the  other  like  the  two  parts  of 
a  pill  box.  This  wall  is  so  impregnated  with  silica  that  it 
is  practically  indestructible,  and  siliceous  skeletons  of  dia- 
toms are  preserved  abundantly  in  certain  rock  deposits. 
They  multiply  by  cell  division  in  a  peculiar  way,  and  some 
of  them  have  been  observed  to  con- 
jugate. 

They  occur  in  such  numbers  in  the 
ocean  that  they  form  a  large  part  of 
the  free-swimming  forms  on  the  sur- 
face of  the  sea,  and  doubtless  showers 
of  the  siliceous  skeletons  are  constant- 
ly falling  on  the  sea  bottom.  There 
are  certain  deposits  known  as  "si- 
liceous earths,"  which  are  simply 
masses  of  fossil  diatoms. 

Diatoms  have  been  variously  placed 
in  schemes  of  classification.  Some 
have  put  them  among  the  Brown 
Algae  because  they  contain  a  brown 
coloring  matter;  others  have  placed 
them  in  the  Conjugate  forms  among 
the  Green  Algae  on  account  of  the 
occasional  conjugation  that  has  been 
observed.  They  are  so  different  from 
other  forms,  however,  that  it  seems 
best  to  keep  them  separate  from  all 
other  Algae. 

176.  Characeae. — These  are  common- 
ly called  "  stoneworts,"  and  are  often 
included  as  a  group  of  Green  Algae, 
as  they  seem  to  be  Thallophytes,  and 
have  no  other  coloring  matter  than 
chlorophyll.  However,  they  are  so  peculiar  that  they  are 
better  kept  by  themselves  among  the  Algae.    They  are  such 


Fig.  231.  A  common  Chara, 
showing  tip  of  main  axis. 
—After  Strasburger. 


THE  GREAT  GROUPS  OF  ALGJE  263 

specialized  forms,  and  are  so  much  more  highly  organized 
than  all  other  Algre,  that  they  will  be  passed  over  here  with 
a  bare  mention.  They  grow  in  fresh  or  brackish  waters, 
fixed  to  the  bottom,  and  forming  great  masses.  The  cylin- 
drical stems  are  jointed,  the  joints  sending  out  circles  of 
branches,  which  repeat  the  jointed  and  branching  habit 
(Fig.  231). 

The  walls  become  incrusted  with  a  deposit  of  lime, 
which  makes  the  plants  harsh  and  brittle,  and  has  sug- 
gested the  name  "  stoneworts."  In  addition  to  the  highly 
organized  nutritive  body,  the  antheridia  and  oogonia  are 
peculiarly  complex,  being  entirely  unlike  the  simple  sex 
organs  of  the  other  Algse. 


18 


CHAPTEE   XVIII 

THALLOPHYTES :  FUNGI 

177.  General  characters. — In  general,  Fungi  include  Thal- 
lopliytes  which  do  not  contain  chlorophyll.  From  this  fact 
it  follows  that  they  can  not  manufacture  food  entirely  out 
of  inorganic  material,  but  are  dependent  for  it  upon  other 
plants  or  animals.  This  food  is  obtained  in  two  general 
ways,  either  (1)  directly  from  the  living  bodies  of  plants  or 
animals,  or  (2)  from  dead  bodies  or  the  products  of  living 
bodies.  In  the  first  case,  in  which  living  bodies  are  at- 
tacked, the  attacking  fungus  is  called  a  parasite^  and  the 
plant  or  animal  attacked  is  called  the  host.  In  the  second 
case,  in  which  living  bodies  are  not  attacked,  the  fungus  is 
called  a  saprophyte.  Some  Fungi  can  live  only  as  parasites, 
or  as  saprophytes,  but  some  can  live  in  either  way. 

Fungi  form  a  very  large  assemblage  of  plants,  much 
more  numerous  than  the  Algae.  As  many  of  the  parasites 
attack  and  injure  useful  plants  and  animals,  producing 
many  of  the  so-called  "  diseases,"  they  are  forms  of  great 
interest.  Governments  and  Experiment  Stations  have  ex- 
pended a  great  deal  of  money  in  studying  the  injurious 
parasitic  Fungi,  and  in  trying  to  discover  some  method  of 
destroying  them  or  of  preventing  their  attacks.  Many  of 
the  parasitic  forms,  however,  are  harmless ;  while  many  of 
the  saprophytic  forms  are  decidedly  beneficial. 

It  is  generally  supposed  that  the  Fungi  are  derived  from 
the  Algae,  having  lost  their  chlorophyll  and  power  of  inde- 
pendent living.  Some  of  them  resemble  certain  Algae  so 
closely  that  the  connection  seems  very  plain:  but  others 
264 


THALLOPHYTES:    FUNGI 


265 


have  been  so  modified  by  their  parasitic  and  saprophytic 
habits  that  they  have  lost  all  likeness  to  the  Algae,  and 
their  connection  with  them  is  very  obscure. 

178.  The  plant  body. — Discarding  certain  problematical 
forms,  to  be  mentioned  later,  the  bodies  of  all  true  Fungi 
are  organized  upon  a  uniform  general  plan,  to  which  they 
can  all  be  referred  (Fig.  232).     A  set  of  colorless  branching 


Fig.  232.  A  diagrammatic  representation  of  Miic<yi\  showing  the  profusely  branching 
mycelium,  and  three  vertical  hyphae  (sporophoree),  sporangia  forming  on  b  and  c. 
—After  Zopp. 


filaments,  either  isolated  or  interwoven,  forms  the  main 
working  body,  and  is  called  the  myceUum.  The  interweav- 
ing may  be  very  loose,  the  mycelium  looking  like  a  delicate 
cobweb ;  or  it  may  be  close  and  compact,  forming  a  felt-like 
mass,  as  may  often  be  seen  in  connection  with  preserved 
fruits.  The  individual  threads  are  called  Injplice  (singular, 
liypha)  or  hyphal  threads.  The  mycelium  is  in  contact  with 
its  source  of  food  supply,  which  is  called  the  substratum. 


2G6 


FLArvT   STUDIES 


From  the  liyphal  threads  composing  the  mycelium  verti- 
cal ascending  branches  arise,  which  are  set  apart  to  produce 
the  asexual  sj^ores,  which  are  scattered  and  produce  new 
mycelia.  These  branches  are  called  ascending  liyplim  or 
sporophores,  meaning  "  spore  bearers." 

Sometimes,  especially  in  the  case  of  parasites,  special 
descending  branches  are  formed,. which  penetrate  the  sub- 
stratum or  host  and  absorb  the  food  material.  These  spe- 
cial absorbing  branches  are  called  haustoria^  meaning  "  ab- 
sorbers." 

Such  a  mycelial  body,  with  its  sporophores,  and  perhaps 
haustoria,  lies  either  upon  or  within  a  dead  substratum  in 
the  case  of  saprophytes,  or  upon  or  within  a  living  plant  or 
animal  in  the  case  of  parasites. 

179.  The  subdivisions. — The  classification  of  Fungi  is  in 
confusion  on  account  of  lack  of  knowledge.  They  are  so 
much  modified  by  their  peculiar  life  habits  that  they  have 
lost  or  disguised  the  structures  which  prove  most  helpful  in 
classification  among  the  Algae.  Four  groups  will  be  pre- 
sented, often  made  to  include  all  the  Fungi,  but  doubtless 
they  are  insufificient  and  more  or  less  unnatural. 

The  constant  termination  of  the  group  names  is  mycetes^ 
a  Greek  word  meaning  "  fungi."  The  prefix  in  each  case  is 
intended  to  indicate  some  important  character  of  the  group. 
The  names  of  the  four  groups  to  be  presented  are  as  follows : 
(1)  Pliycomycetes  ("  Alga-Fungi "),  referring  to  the  fact 
that  the  forms  plainly  resemble  the  Algae  ;  (2)  Ascomycetes 
("  Ascus-Fungi ") ;  (3)  ^cicUomycetes  ("^^cidium-Fungi  ") ; 
(4)  Basidiomycetes  ("  Basidium-Fungi ").  Just  what  the 
prefixes  ascus,  ceciditim,  and  iasidiuni  mean  will  be  ex- 
plained in  connection  with  the  groups.  The  last  three 
groups  are  often  associated  together  under  the  name  My- 
comycetes^  meaning  "  Fungus-Fungi,"  to  distinguish  them 
from  the  Phycomycetes,  or  "  Alga-Fungi,"  referring  to  the 
fact  that  they  do  not  resemble  the  Alga?,  and  are  only  like 
themselves. 


THALLOPIIYTES:    FUNGI  267 

One  of  the  ordinary  life  processes  which  seems  to  be 
seriously  interfered  with  by  the  saprophytic  and  parasitic 
habit  is  the  sexual  process.  At  least,  while  sex  organs 
and  sexual  spores  are  about  as  evident  in  Phycomycetes 
as  in  Algae,  they  are  either  obscure  or  wanting  in  the 
Mycomycete  groups. 

1.   Phycomycetes  (Alga-Ftcngi) 

180.  Saprole^a. — This  is  a  group  of  "water-moulds," 
with  aquatic  habit  like  the  Algse.  They  live  upon  the  dead 
bodies  of  water  plants  and  animals  (Fig.  233),  and  some- 
times attack  living  fish,  one  kind  being  very  destructive 
to  young  fish  in  hatcheries.  The  hyph^  composing  the 
mycelium  are  coenocytes,  as  in  the  Siphon  forms. 

Sporangia  are  organized  at  the  ends  of  branches  by 
forming  a  partition  wall  separating  the  cavity  of  the  tip 
from  the  general  cavity  (Fig.  233,  B).  The  tip  becomes 
more  or  less  swollen,  and  within  it  are  formed  numerous 
biciliate  zoospores,  which  are  discharged  into  the  water 
(Fig.  233,  C),  swim  about  for  a  short  time,  and  rapidly  form 
new  mycelia.  The  process  is  very  suggestive  of  Claclojyhora 
and  Vaucheria.  Oogonia  and  antheridia  are  also  formed 
at  the  ends  of  the  branches  (Fig.  233,  F)^  much  as  in  Vau- 
cheria. The  oogonia  are  spherical,  and  form  one  and  some- 
times many  eggs  (Fig.  233,  D,  E).  The  antheridia  are 
formed  on  branches  near  the  oogonia.  An  antheridium 
comes  in  contact  with  an  oogonium,  and  sends  out  a  deli- 
cate tube  which  pierces  the  oogonium  wall  (Fig.  233,  F). 
Through  this  tube  the  contents  of  the  antheridium  pass, 
fuse  with  the  egg.,  and  a  heavy-walled  oospore  or  resting 
spore  is  the  result. 

It  is  an  interesting  fact  that  sometimes  the  contents  of 
an  antheridium  do  not  enter  an  oogonium,  or  antheridia 
may  not  even  be  formed,  and  still  the  agg^  without  fertiliza- 
tion, forms  an  oospore  which  can  germinate.    This  peculiar 


268 


PLANT   STUDIES 


habit  is  called  parthenogenesis^  which  means  reproduction 
by  an  Qgg  without  fertilization. 


Fig. 233.  A  common  water  mould  (Saprolegnia):  A,  a  fly  from  which  mycelial  fila- 
ments of  the  parasite  are  growing;  B,  tip  of  a  branch  organized  as  a  sporangium; 
C,  sporangium  discharging  biciliate  zoospores;  F,  oogonium  with  antheridium  in 
contact,  the  tube  having  penetrated  to  the  egg;  Z>  and  B,  oogonia  with  several 
eggs.— J.- C  after  Thuret,  Z>-i^  after  DeBary. 


181.  Mucor. — One  of  the  most  common  of  the  Mucors,  or 
"black  moulds,"  forms  white  furry  growths  on  damp  bread, 
preserved  fruits,  manure  heaps,  etc.  It  is  therefore  a 
saprophyte,  the  coenocytic  mycelium  branching  extensively 
through  the  substratum  (Fig.  234). 


TIIALLOniYTES:    FUNGI 


269 


Erect  sporophores  arise  from  it  in  abundance,  and  at 
the  top  of  each  sporophore  a  globular  sporangium  is  formed, 
within  which  are  numerous  small  asexual  spores  (Figs.  235, 


Fig.  234. 


Diagram  showing  mycelium  and  sporophores  of  a  common  Mvcor.- 

MOORB. 


I 


236).  The  sporangium  wall  bursts  (Fig.  237),  the  light  spores 
are  scattered  by  the  wind,  and,  falling  upon  a  suitable  sub- 
stratum, germinate  and 
form  new  mycelia.  It  is 
evident  that  these  asex- 
ual spores  are  not  zoo- 
spores, for  there  is  no 
water  medium  and  swim- 
ming is  impossible.  This 
method  of  transfer  being 
impossible,  the  spores  are 
scattered  by  currents  of 
air,  and  must  be  corre- 
spondingly light  and  pow- 
dery.     They   are    usually    ^    ^„.    ^      .  .    , ,, 

*'  *'        .  '^      Fig.  235.    Forming  sporangia  of  Mucor,  show- 

Spoken    of    simply   as  Ing  the  swollen  tip  of  the  sporophore  {A), 

"  spores,"     without     any         ''"^  ^  ^*^*"  ^^"^^  (^^^  "'  '^'"^^  "  ""'^  ^^ 

*^  formed  separating  the  sporangium  from 

prenx.  the  rest  of  the  body.— Moore. 


270 


PLANT   STUDIES 


While  the  ordinary  method  of  reproduction  through  the 
growing  season  is  by  means  of  these  rapidly  germinating 
spores,  in  certain  conditions  a  sexual  process  is  observed, 
by  which  a  heavy-walled  sexual  spore  is  formed  as  a  resting 
spore,  able  to  outlive  unfavorable  conditions.  Branches 
arise  from  the  hyphse  of  the  mycelium  just  as  in  the  forma- 


FiG.  236.  Mature  eporangium  of  Mucor,  showing 
the  wall  {A),  the  numerous  spores  (C).  and 
the  columella  (5)— that  is,  the  partition  wall 
pushed  up  into  the  cavity  of  the  sporangium. 
— Moore. 


Fig.  237.  Bursted  sporangium  of 
Mncor,  the  ruptured  wall  not 
being  shown,  and  the  loose 
spores  adhering  to  the  colu- 
mella.—Moore. 


tion  of  sporophores  (Fig.  238).  Two  contiguous  branches 
come  in  contact  by  their  tips  (Fig.  238,  A)^  the  tips  are  cut 
off  from  the  main  coenocytic  body  by  partition  walls  (Fig. 
238,  E)^  the  walls  in  contact  disorganize,  the  contents  of 
the  two  tip  cells  fuse,  and  a  heavy-walled  sexual  spore  is 
the  result  (Fig.  238,  C).  It  is  evident  that  the  process  is 
conjugation,  suggesting  the  Conjugate  forms  among  the 


TllALLOl'HYTES:   FUNGI 


271 


Algae  ;  that  the  sexual  spore  is  a  zygote  ;  and  that  the  two 
pairing  tip  cells  cut  off  from  the  main  body  by  partition 
walls  are  gametangia.     Mucor,  therefore,  is  isogamous. 


Fig.  238.  Sexual  reproduction  of  Mvcor.  showing  tips  of  sex  branches  meetincr  ( i) 
the  two  gametangia  cut  off  by  partition  walls  (B),  and  the  heavy-walled  zy-ote 
( 6').— Caldwell. 


182.  Peronospora.— These  are  the  "  downy  mildews,"  very 
common  parasites  on  seed  plants  as  hosts,  one  of  the  most 
common  kind  attacking  grape  leaves.  The  mycelium  is 
coenocytic  and  entirely  internal,  ramifying  among  the  tis- 
sues within  the  leaf,  and  piercing  the  living  cells  with  haus- 
toria  which  rapidly  absorb  tlieir  contents  (Fig.  239).  The 
presence  of  the  parasite  is  made  known  by  discolored  and 


272 


PLANT   STUDIES 


finally  deadened  spots  on  the  leaves,  where  the  tissues  have 
been  killed. 

From  this  internal  mycelium  numerous  sporophores 
arise,  coming  to  the  surface  of  the  host  and  securing  the 
scattering  of  their 
spores,  which  fall 
upon  other  leaves 
and  germinate,  the 
new  mycelia  pene- 
trating among  the 
tissues  and  begin- 
ning their  ravages. 
The  sporophores,  af- 
ter rising  above  the 
surface   of  the  leaf, 

branch  freely ;  and  many  of  them  rising  near  together, 
they  form  little  velvety  patches  on  the  surface,  suggesting 
the  name  "  downy  mildew." 

h  c 


Fig.  239.  A  branch  of  Peronospora  in  contact  with 
two  cells  of  a  host  plant,  and  sending  into  them 
its  large  hauetoria.— After  DeBart. 


Fig.  240.  Peronospora,  one  of  the  Phycomycetes,  showing  at  a  an  oogonium  (o)  con- 
taining an  egg,  and  an  antheridium  («)  in  contact;  at  b  the  antheridial  tube  pene- 
trating the  oogonium  and  discharging  the  contents  of  the  antheridium  into  the 
egg;  at  c  the  oogonium  containing  the  oospore  or  resting  spore. — After  DeBary. 


In  certain  conditions  special  branches  arise  from  the 
mycelium,  which  organize  antheridia  and  oogonia,  and 
remain  within  the  host  (Fig.  240).  The  oogonium  is  of 
the  usual  spherical  form,  organizing  a  single  Qgg.     The  an- 

M.0011EGE  LIBRARY. 


TIIALLOPHYTES :  FUNGI  273 

theridium  comes  in  contact  with  the  oogonium,  puts  out  a 
tube  which  pierces  the  oogonium  wall  and  enters  the  egg, 
into  which  the  contents  of  the  antheridium  are  discharged, 
and  fertilization  is  effected.  The  result  is  a  heavy-walled 
oospore.  As  the  oospores  are  not  for  immediate  germina- 
tion, they  are  not  brought  to  the  surface  of  the  host  and 
scattered,  as  are  the  asexual  spores.  When  they  are  ready 
to  germinate,  the  leaves  bearing  them  have  perished  and 
the  oospores  are  liberated. 

183.  Conclusions. — The  coenocytic  bodies  of  the  whole  group 
are  very  suggestive  of  the  Siphon  forms  among  Green  Alg^e, 
as  is  also  the  method  of  forming  oogonia  and  antheridia. 

The  water-moulds,  Saprolegnia  and  its  allies,  have  re- 
tained the  aquatic  habit  of  the  Algae,  and  their  asexual 
spores  are  zoospores.  Such  forms  as  Mucor  and  Perono- 
spora,  however,  have  adapted  themselves  to  terrestrial  con- 
ditions, zoospores  are  abandoned,  and  light  spores  are  de- 
veloped which  can  be  carried  about  by  currents  of  air. 

In  most  of  them  motile  gametes  are  abandoned.  Even 
in  the  heterogamous  forms  sperms  are  not  organized  within 
the  antheridium,  but  the  contents  of  the  antheridium  are 
discharged  through  a  tube  developed  by  the  wall  and  pene- 
trating the  oogonium.  It  should  be  said,  however,  that  a 
few  forms  in  this  group  develop  sperms,  which  make  them 
all  the  more  alga-like. 

They  are  both  isogamous  and  heterogamous,  both  zygotes 
and  oospores  being  resting  spores.  Taking  the  characters 
all  together,  it  seems  reasonably  clear  that  the  Phycomycetes 
are  an  assemblage  of  forms  derived  from  Green  Algae  (Chlo- 
rophyceae)  of  various  kinds. 

2.  AscoMYCETES  {Ascus-  ov  Sac-Fu7igi) 

184.  Mildews. — These  are  very  common  parasites,  growing 
especially  upon  leaves  of  seed  plants,  the  mycelium  spread- 
ing over  the  surface  like  a  cobweb.     A  very  common  mil- 


274 


PLANT   STUDIES 


dew,  Microsphcera^  grows  on  lilac  leaves,  which  nearly  al- 
ways show  the  whitish  covering  after  maturity  (Fig.  241). 
The  branching  hyphae  show  numerous  partition  walls,  and 
are  not  coenocytic  as  in  the  Phycomycetes.  Small  disk-like 
haustoria  penetrate  into  the  superficial  cells  of  the  host, 
anchoring  the  mycelium  and  absorbing  the  cell  contents. 

Sporophores  arise,  which  form  asexual  spores  in  a  pe- 
culiar way.  The  end  of  the  sporophore  rounds  off,  almost 
separating  itself  from  the  part  below,  and  becomes  a  spore 
or  spore-like  body.      Below  this  another  organizes  in  the 

same  way,  then  another,  until 
a  chain  of  spores  is  developed, 
easily  broken  apart  and  scat- 
tered by  the  Avind.  Falling 
upon  other  suitable  leaves, 
they  germinate  and  form  new 
mycelia,  enabling  the  fungus 
to  spread  rapidly.  This  meth- 
od of  cutting  a  branch  into 
sections  to  form  spores  is 
called  abstriction^  and  the 
spores  formed  in  this  way 
are  called  conidia^  or  conidi' 
ospores  (Fig.  243,  B). 

At  certain  times  the  myce- 
lium develops  special  branches 
which  develo^J  sex  organs,  but 
they  are  seldom  seen  and  may 
not  always  occur.  An  oogo- 
nium and  an  antheridium,  of 
the  usual  forms,  but  probably 
without  organizing  gametes, 
come  into  contact,  and  as  a 
structure  is  developed — the  ascocarp^ 
spore  fruit."     These  ascocarps  ap- 


FiG.  241.  Lilac  leaf  covered  with  mil- 
dew {Microsjyhoera),  the  shaded  re- 
gions representing  the  mycelium, 
and  the  black  dots  the  ascocarps.— 
S.  M.  Coulter. 


result  an  elaborate 

sometimes  called  the  " 

pear  on  the  lilac  leaves  as  minute  dark  dots,  each  one  being 


THALLOPHYTES:    FUNGI 


275 


a  little  sphere,  which  suggested  the  name  Microsphcera 
(Fig.  241).  The  heavy  wall  of  the  ascocarp  bears  beauti- 
ful branching  hair-like  appendages  (Fig.  242). 

Bursting  the  wall  of  this  spore  fruit  several  very  delicate, 
bladder-like  sacs  are  extruded,  and  through  the  transparent 
wall  of  each  sac  there  may 
be  seen  several  spores  (Fig. 
242).  The  ascocarp,  there- 
fore, is  a  spore  case,  just  as 
is  the  cystocarp  of  the  Red 
Algge  (§  174).  The  delicate 
sacs  within  are  the  asci^  a 
word  meaning  "  sacs,"  and 
each  ascus  is  evidently  a 
mother  cell  within  which 
asexual  spores  are  formed. 
These  spores  are  distin- 
guished from  other  asexual 
spores  by  the  name  asco- 
spore. 

It  is  these  peculiar  moth- 
er cells,  or  asci,  which  give 

name  to  the  group,  and  an  Ascomycete,  Ascus-fungus,  or 
Sac-fungus,  is  one  which  produces  spores  in  asci ;  and  an 
ascocarp  is  a  spore  case  which  contains  asci. 

In  the  mildews,  therefore,  there  are  tAvo  kinds  of  asexual 
spores  :  (1 )  conidia,  formed  from  a  hyphal  branch  by  abstric- 
tion,  by  which  the  mycelium  may  spread  rapidly ;  and  (2) 
ascospores,  formed  in  a  mother  cell  and  protected  by  a  heavy 
case,  so  that  they  may  bridge  over  unfavorable  conditions, 
and  may  germinate  when  liberated  and  form  new  mycelia. 
The  resting  stage  is  not  a  zygote  or  an  oospore,  as  in  the 
Algae  and  Phycomycetes,  no  sexual  spore  probably  being 
formed,  l)ut  a  lieavy-walled  ascocarp. 

185.  Other  forms. — The  mildews  have  been  selected  as  a 
simple  illustration  of  Ascomycetes,  but  the  group  is  a  very 


^"^v 


Fig.  242.  Ascocarp  of  the  lilac  mildew, 
showing  branching  appendages  and 
two  asci  protruding  from  the  ruptured 
wall  and  containing  ascospores.— S. 
M.  Coulter. 


276 


PLANT   STUDIES 


large  one,  and  contains  a  great  variety  of  forms.  All  of 
them,  however,  produce  spores  in  asci,  but  the  asci  are  not 
always  inclosed  by  an  ascocarp.  Here  belong  the  common 
blue  mould  (Fenicillium)  found  on  bread,  fruit,  etc.,  in 
which  stage  the  branching  chains  of  conidia  are  very  con- 
spicuous (Fig.  243) ;  the  truffle-fungi,  upon  whose  subter- 


FiG.  243.  Penicillium,  a  common  mould  :  A,  mycelium  with  numerous  branching: 
eporophores  bearing  conidia  ;  B,  apex  of  a  sporophore  enlarged,  showing  branch- 
ing and  chains  of  conidia.— After  Brefeld. 


ranean  mycelia  ascocarps  develop  which  are  known  as 
"  truffles  " ;  the  black  fungi,  which  form  the  diseases  known 
as  "  black  knot "  of  the  plum  and  cherry,  the  "  ergot "  of 
rye  (Fig.  244),  and  many  black  wart-like  growths  upon  the 
bark  of  trees ;  other  forms  causing  "  witches'-brooms  "  (ab- 
normal growths  on  various  trees),  "peach  curl,"  etc.,  the 
cuprfungi  (Figs.  245,  246),  and  the  edible  morels  (Fig.  247). 


ft] 


THALLOPHTTES :  FUNGI 


'2-i7 


Fig.  244.  Head  of  rye  attacked  by  "  er- 
got" (a),  peculiar  grain-like  masses 
replacing  the  grains  of  rye  ;  also  a 
mass  of  "ergot"  germinating  to 
form  spores  (&).— After  Tulasne. 


Fig.  246.  A  cup-fungus  (Pitya)  grow- 
ing on  a  spruce  (Pic^a).  —  After 
Kehm. 


In  some  of  these  forms  the  ascocarp  is  completely  closed, 
as  in  the  lilac  mildew  ;  in  others  it  is  flask-shaped  ;  in 
others,  as  in  the  cup-fungi,  it  is  like  a  cup  or  disk  ;  but  in 
all  the  spores  are  inclosed  by  a  delicate  sac,  the  ascus. 


278 


PLANT^TDDIES 


Here  must  probably  be  included  the  yeast-fungi  (Figc 
248),  so^commonly  used  to  excite  alcoholic  fermentatioUc 


Fig.  247.  The  common  edible  morel  (Morchella 
esculenta).  The  structure  shown  and  used 
represents  the  ascocarp,  the  depressions  of 
whose  surface  are  lined  with  asci  contain- 
ing ascospores.— After  Gibson. 


Fig.  248.  Yeast  cells,  repro- 
ducing by  budding,  and 
forming  chains.— Land. 


The  "  yeast  cells  "  seem  to  be  conidia  having  a  peculiar  bud- 
ding method  of  multiplication,  and  the  remarkable  power 
of  exciting  alcoholic  fermentation  in  sugary  solutions. 

3.  ^ciDiOMYCETES  (^cidium-Fuugi) 

186.  General  characters.— This  is  a  large  group  of  very 
destructive  parasites  known  as  "  rusts  "  and  "  smuts."  The 
rusts  attack  particularly  the  leaves  of  higher  plants,  pro- 
ducing rusty  spots,  the  wheat  rust  probably  being  the  best 
known.  The  smuts  especially  attack  the  grasses,  and  are 
very  injurious  to  cereals,  producing  in  the  heads  of  oats, 
barley,  wheat,  corn,  etc.,  the  disease  called  smut. 


THALLOPIIYTES:    FUNGI 


279 


In  some  forms  an  obscure  sexual  process  has  been  de- 
scribed, but  it  is  beyond  the  reach  of  ordinary  observation. 
The  ^cidiomycetes  do  not  form  an  independent  and  nat- 
ural group,  but  are  now  generally  placed  under  the  Ba- 
sidiomycetes,  but  they  are  so  unlike  the  ordinary  forms  of 
that  group  that  they  are  here  kept  distinct  for  convenience. 

Most  of  the  forms  are  NQvy polymorphic — that  is,  a  plant 
assumes  several  dissimilar  appearances  in  the  course  of  its 
life  history.  These  phases  are  often  so  dissimilar  that  they 
have  beeK  described  as  different  plants.  This  polymorphism 
is  often  further  complicated  by  the  appearance  of  different 
phases  upon  entirely  different  hosts.  For  example,  the 
wheat-rust  fungus  in  one  stage  lives  on  wheat,  and  in  an- 
other on  barberry. 

187.  Wheat  rust. — This  is  one  of  the  few  rusts  whose  life 
histories  have  been  traced,  and  it  may  be  taken  as  an  illus- 
tration of  the  group. 

The  mycelium  of  the  fungus  is  found  ramifying  among 
the  leaf  and  stem  tissues  of  the  wheat.  While  the  wheat  is 
growing  this  mycelium  sends  to  the  surface  numerous  spo- 


Fiu.'2-iy.  Wheat  rnst,  showing  sporophores  breaking  through  the  tissues  of  the  host 
and  bearing  summer  spores  (uredosporee).— After  H.  Marshall  Ward. 


rophores,  each  bearing  at  its  apex  a  reddish  spore  (Fig.  240). 
As  the  spores  occur  in  great  numbers  they  form  the  rusty- 
looking  lines  and  spots  wliich  give  name  to  the  disease. 
The  spores  are  scattered  by  currents  of  air,  and  falling  upon 
other  plants,  germinate  very  promptly,  thus  spreading  the 
19  " 


280 


PLANT   STUDIES 


disease  with  great  rapidity  (Fig.  250).  Once  it  was  thought 
that  this  completed  the  life  cycle,  and  the  fungus  received 
the  name  Uredo.     When  it  was  known  that  this  is  hut  one 


Fig.  250.    Wheat  rust,  showing  a  young  hypha  forcing  its  way  from  the  surface  of  a 
leaf  down  among  the  nutritive  cells.— After  H.  Marshall  Ward. 

stage  in  a  polymorphic  life  history  it  was  called  the  Ure do- 
stage,  and  the  spores  uredospores^  sometimes  "summer 
spores." 


Fig.  251.    Wheat  rust,  showing  the  winter  spores  (teleutospores).— After 
H.  Marshall  Ward. 

Toward  the  end  of  the  summer  the  same  mycelium 
develops  sporophores  which  hear  an  entirely  different  kind 
of  spore  (Fig.  251).    It  is  two-celled,  with  a  very  heavy  hlack 


THALLOPHYTES :  FUNGI 


281 


wall,  and  forms  what  is  called  the  "  black  rust,"  which  ap- 
pears late  in  the  summer  on  wheat  stubble.  These  spores 
are  the  resting  spores,  which  last  through  the  winter  and 
germinate  in  the  following  spring.  They  are  called  teleuto- 
spores,  meaning  the  "  last  spores  "  of  the  growing  season. 
They  are  also  called  "  winter  spores,"  to  distinguish  them 
from  the  uredospores  or  "  summer  spores."  At  first  tliis 
teleutospore-bearing  mycelium  was  not  recognized  to  be 
identical  with  the  uredospore-bearing  mycelium,  and  it  was 
called  Puccinia.  This  name  is  now 
retained  for  the  whole  polymorphous 
plant,  and  wheat  rust  is  Puccinia 
graminis.  This  mycelium  on  the 
wheat,  with  its  summer  spores  and 
winter  spores,  is  but  one  stage  in 
the  life  history  of  wheat  rust. 

In  the  spring  the  teleutospore 
germinates,  each  cell  developing  a 
small  few-celled  filament  (Fig.  252). 
From  each  cell  of  the  filament  a 
little  branch  arises  which  develops 
at  its  tip  a  small  spore,  called  a  spo- 
ridhim,  which  means  "  spore-like." 
This  little  filament,  which  is  not  a 
parasite,  and  which  bears  sporidia, 
is  a  second  phase  of  the  wheat  rust, 
really  the  first  phase  of  the  growing 
season. 

The  sporidia  are  scattered,  fall 
upon  barberry  leaves,  germinate,  and 
develop  a  mycelium  which  spreads 

through  the  leaf.  This  mycelium  produces  sporophores 
which  emerge  on  the  under  surface  of  the  leaf  in  the 
form  of  chains  of  reddish-yellow  conidia  (Fig.  253).  These 
chains  of  conidia  are  closely  packed  in  cup-like  receptacles, 
and  these  reddish-yellow  cup-like  masses  are  often  called 


Fig.  252.  Wheat  rust,  show- 
ing a  teleutospore  germina- 
ting and  forming  a  short  fil- 
ament, from  four  of  whose 
cells  a  spore  branch  arises, 
the  lowest  one  bearing  at 
its  tip  a  sporidium.— After 
H.  Marshall  Ward. 


282 


PLANT   STUDIES 


"cluster-cups."     This  mycelium  on  the  barberry,  bearing 
cluster-cups,  was  thought  to  be  a  distinct  plant,  and  was 

called  ^cidium.  The 
name  now  is  applied  to 
the  cluster-cups,  which 
are  called  cecidia^  and 
the  conidia-like  spores 
which  they  produce  are 
known  as  CBcidiospores. 

It  is  the  aecidia  which 
give  name  to  the  group, 
and  ^cidiomycetes  are 
those  Fungi  in  whose 
life  history  aecidia  or 
cluster-cups  appear. 

The  aecidiospores  are 
scattered  by  the  wind, 
fall  upon  the  spring 
wheat,  germinate,  and 
develop  again  the  myce- 
lium which  produces  the 
rust  on  the  wheat,  and 
so  the  life  cycle  is  com- 
pleted. There  are  thus 
at  least  three  distinct 
stages  in  the  life  history 
of  wheat  rust.  Begin- 
ning with  the  growing 
season  they  are  as  fol- 
lows :  (1)  The  phase  bear- 
ing the  sporidia,  which 
is  not  parasitic ;  (2)  the 
aecidium  phase,  parasitic 
on  the  barberry;  (3)  the  uredo-teleutospore  phase,  para- 
sitic on  the  wheat. 

In  this  life  cycle  at  least  four  kinds  of  asexual  spores 


THALLOPHYTES:    FUNGI 


283 


appear  :  (1)  sporidia,  which  develop  the  stage  on  the  barber- 
ry ;  (2)  (Bcidios2)ores^  which  develop  the  stage  on  the  wheat ; 
{Z)uredo spores, which,  repeat  the  mycelium  on  the  wheat ;  (4) 
teleutospores,\\h\ch  last  through  the  winter,  and  in  the  spring 
produce  the  stage  bearing  sporidia.  It  should  be  said  that 
there  are  other  structures  of  this  plant  produced  on  the  bar- 
berry (Fig.  53),  but  they  are  too  uncertain  to  be  included  here. 
The  barberry  is  not  absolutely  necessary  to  this  life  cycle. 
In  many  cases  there  is  no  available  barberry  to  act  as  host, 
and  the  sporidia  germinate  directly  upon  the  young  wheat, 
forming  the  rust-producing  mycelium,  and  the  cluster-cup 
stage  is  omitted. 


Fig.  254.    Two  species  of  "cedar  apple  "  ( Gt/fn}}0$}X)ranr/imn).  both  on  the  common 
juniper  {Junij^rus  Virginiana).—A  after  Faklow,  B  after  Engler  and  Prantl. 


188.  Other  rusts. — Many  rusts  have  life  histories  similar 
to  that  of  the  wheat  rust,  in  others  one  or  more  of  the 
stages  are  omitted.     In  very  few  have  the  stages  been  con- 


284 


PLANT   STUDIES 


nected  together,  so  that  a  mycelium  bearing  iiredospores  is 
called  a  Ureclo^  one  bearing  teleutospores  a  Puccinia^  and 
one  bearing  ascidia  an  ^Ecidium, ;  but  what  forms  of  Uredo, 
Puccinia^  and  uEcidium  belong  together  in  the  same  life 
cycle  is  very  difficult  to  discover. 

Another  life  cycle  which  has  been  discovered  is  in  con- 
nection with  the  "  cedar  apples ''  which  appjear  on  red 
cedar  (Fig.  254).  In  the  spring  these  diseased  growths  be- 
come conspicuous,  especially  after  a  rain,  when  the  jelly- 
like masses  containing  the  orange-colored  spores  swell. 
This  corresponds  to  the  phase  which  produces  rust  in 
wheat.  On  the  leaves  of  apple  trees,  wild  crab,  hawthorn, 
etc.,  the  fficidium  stage  of  the  same  parasite  develops. 

4.  Basidiomycetes  {Basidiuni^Fungi). 
189.  General  characters, — This  group  includes  the  mush- 
rooms, toadstools,  and  puffballs.     They  are  not  destructive 

parasites,  as  are  many 
forms  in  the  preceding 
groups,  but  mostly  harm- 
less and  often  useful  sap- 
rophytes. They  must 
also  be  regarded  as  the 
most  highly  organized  of 
the  Fungi.  The  popular 
distinction  between  toad- 
stools and  mushrooms  is 
not  borne  out  by  botan- 
ical characters,  toadstool 
and  mushroom  being  the 
same  thing  botanically, 
and  forming  one  group, 
puffballs  forming  an- 
other. 

As  in  ^cidiomycetes, 

Fig.  255.     The  common  edible  mushroom,  i 

Agaricus  campestris.- After  Gibson.  an  obsCUrC  SexuaJ  prOCCSS 


THALLOPHYTES:   FUNGI 


285 


is  reported.  The  life  history  seems  simple,  but  this  appar- 
ent simplicity  may  represent  a  very  complicated  history. 
The  structure  of  the  common  mushroom  (Agaricus)  will 
serve  as  an  illustration  of  the  group  (Fig.  255). 

190.  A  common 
mushroom.  —  The 
mycelium,  of  white 
branching  threads, 
spreads  extensively 
through  the  decay- 
ing substratum, 
and  in  cultivated 
forms  is  spoken  of 
as  the  "  spawn." 
Upon  this  myce- 
lium little  knob- 
like protuberances 
begin  to  arise,  gro w- 
ing  larger  and 
larger,  until  they 
are  organized  into 
the  so-called 
*'  mushrooms." 
The  real  body  of 
the  plant  is  the 
white  thread  -  like 
mycelium,  while 
the  "  mushroom  " 
part  seems  to  rep- 
resent a  great  num- 
ber of  sporophores 
organized  together 
to  form  a  single 
complex  spore- 
bearing  structure. 
The  mushroom 


Fig.  256.  A  common  Agaricus  :  A,  section  throiurh  one 
side  of  pileiis,  showing  sections  of  the  pendent  gills; 
B,  section  of  a  gill  more  enlarged,  showing  the  cen- 
tral tissue,  and  the  broad  border  formed  by  the  ba- 
sidia:  C,  still  more  enlarged  section  of  one  side  of 
a  gill,  showing  the  club-shaped  basidia  standing  at 
right  angles  to  the  surface,  and  sending  out  a  pair 
of  small  branches,  each  of  which  bears  a  single  ba- 
eidiospore.— After  Sachs. 


ii 


i  I 


^  2 

-I 


.t 


^/tT'  ' 


-«»  </ 


THALLOPHYTES:    FUNGI 


287 


has  a  stalk-like  portion,  the  stipe,  at  the  base  of  which  the 
slender  mycelial  threads  look  like  white  rootlets ;  and  an 
expanded,  umbrella-like  top  called  the  pileus.  From  the 
under  surface  of  the  pileus  there  hang  thin  radiating  plates, 
ov  gills  (Fig.  255).  Each  gill  is  a  mass  of  interwoven  fila- 
ments (hyphge),  whose  tips  turn  toward  the  surface  and 
form  a  compact  layer  of  end  cells  (Fig.  256).     These  end 


Fig.  260.   A  bracket  fungus  {Polyporus^  <rrowing  on  the  trunk  of  a  red  oak. 
Caldwell. 


cells,  forming  the  surface  of  the  gill,  are  club-shaped,  and 
are  called  hasidia.  From  the  broad  end  of  each  basidium 
two  or  four  delicate  branches  arise,  each  bearing  a  minute 
spore,  very  much  as  the  sporidia  appear  in  the  wheat  rust. 


288 


PLANT   STUDIES 


These  spores,  called  basidiosjjores,  shower  down  from  the 
gills  when  ripe,  germinate,  and  produce  new  mycelia.  The 
peculiar  cell  called  the  basidium  gives  name  to  the  group 
Basidiomycetes. 

191.  Other  forms. — Mushrooms  display  a  great  variety  of 
form  and  coloration,  many  of  them  being  very  attractive 


Fig.  261. 


A  toadstool  of  the  bracket  form  which  has  grown  about  blades  of  grass 
without  interfering  with  their  activity.— Caldwell. 


(Figs.  257,  258,  259).  The  "  pore-fungi  "  have  pore-like  de- 
pressions for  their  spores,  instead  of  gills,  as  in  the  very 
common  "bracket-fungus"  {Polyporiis)^  which  forms  hard 
shell-like  outgrowths  on  tree-trunks  and  stumps  (Figs.  260, 


% 


"K'xj^-^ 


Fig.  262.  The  common  edible  Boletus  (B.  edu- 
iis),  in  which  the  gills  are  replaced  by 
pores.— Alter  (iiBsoN. 


Fig.  263.  Another  edible  Boletus  (B.  stro- 
Mlaceus).— After  Gibson. 


Fig. 264.   The  common  edible  "coral  fun- 
gus" {Clavaria).— After  Gibson. 


1 


Fig.  2G5.  Hydtuim  repandum,  in  which  gills 
are  rephiced  by  spinous  processes  ;  edi- 
ble.—After  Gibson. 


290 


PLANT   STUDIES 


261),  and  the  mnshroom-like  Boleti  (Figs.  262,  263).  The 
"ear-fungi"  form  gelatinous,  dark-brown,  shell-shaped 
masses,  and  the  ''  coral  fungi "  resemble  branching  corals 
(Fig.  264).      The  Hydnum  forms  have  spinous    processes 

instead  of  gills  (Fig. 
265).  The  puffballs  or- 
ganize globular  bodies 
(Fig.  266),  within  which 
the  spores  develop,  and 
are  not   liberated  until 


ripe; 


and    with    them 


belong  also  the  "bird's 
nest  fungus,''  the  "  earth 
star,"  the  ill-smelling 
"stink-horn,"  etc. 

OTHER  THALLOPHYTES 
WITHOUT  CHLOROPHYLL 


192.   Slime -moulds. — 

These  perplexing  forms, 
named  Myxomycetes^  do 
not  seem  to  be  related 
to  any  group  of  plants, 
and  it  is  a  question 
wdiether  they  are  to  be  regarded  as  plants  or  animals.  The 
working  body  is  a  mass  of  naked  protoplasm  called  a  plas- 
modiimi^  suggesting  the  term  "  slime,"  and  slips  along  like 
a  gigantic  amoeba.  They  are  common  in  forests,  upon 
black  soil,  fallen  leaves,  and  decaying  logs,  the  slimy  yel- 
low or  orange  masses  ranging  from  the  size  of  a  pinhead 
to  as  large  as  a  man's  hand.  They  are  saprophytic,  and 
are  said  to  engulf  food  as  do  the  amoebas.  So  suggestive 
of  certain  low  animals  is  this  body  and  food  habit  that 
slime-moulds  have  also  been  called  Mycetozoa  or  "  fungus- 
animals." 


Fig.  266.    Pnffballs,  in  which  the  basidia  and 
spores  are  inclosed  ;  edible. — After  Gibson. 


TIIALLOPllYTES:    FUNGI 


291 


In  certain  conditions,  however,  these  slimy  bodies  come 
to  rest  and  organize  most  elaborate  and  often  very  beau- 
tiful sporangia,  full  of  spores  (Fig.  267).  These  varied 
and  easily  preserved   sporangia   are   used   to  classify  the 


Fig.  267.  Three  common  slime  moulds  (Myxomycetes)  on  decaying  wood  :  to  the 
left  above,  groups  of  the  sessile  sporangia  of  Trichia  ;  to  the  right  above,  a  group 
of  the  stalked  sporangia  of  Stemonitis,  with  remnant  of  old  Plasmodium  at  base  ; 
below,  groups  of  sporangia  of  Hemiarcyna,  with  a  Plasmodium  mass  at  upper 
left  hand.— GoLDBERGER. 


forms.  Slime-moulds,  or  "slime-fungi,"  therefore,  seem 
to  have  animal-like  bodies  which  produce  plant-like  spo- 
rangia. 

193.  Bacteria.— These  are  the  "  Fission-Fungi,"  or  Schizo- 
mycetes,  and  are  popularly  known  as  "bacteria,"  "bacilli," 
"  microbes,"  "  germs,"  etc.  They  are  so  important  and  pe- 
culiar in  their  life  habits  that  their  study  has  developed  a 
special  branch  of  botany,  known  as  "Bacteriology."  In 
many  ways  they  resemble  the  Cyanophyceae,  or  "  Fission- 
Algae,"  so  closely  that  they  are  often  associated  with  them 
in  classification  (see  §  IG'2). 


Fig.  268.  A  group  of  Bacteria,  the  bodies  being  black,  and  bearing  motile  cilia  in 
various  was's.  .4,  the  two  to  the  left  the  common  hay  Bacillus  (B.  snbtilis),  'he 
one  to  the  right  a  Spirillmn  ;  B.  a  Coccus  form  (Planocoecus);  C,  Z>,  B,  species  of 
Pseudomonas  :  F,  G,  species  of  Bacillus,  F  being  that  of  typhoid  fever;  H,  Micro- 
spira  ;  J,  K,  L,  M,  species  of  SpiriUum.— After  Englek  and  Pkantl. 


THALLOPHYTES:  FUNGI  293 

They  are  the  smallest  known  living  organisms,  the  one- 
celled  form  which  develops  on  cooked  potatoes,  bread,  milk, 
meat,  etc.,  forming  a  blood-red  stain,  having  a  diameter  of 
but  0.0005  mm.  (-g^oJiro  i^-)-  They  are  of  various  forms 
(Fig.  268),  as  Coccus  forms,  single  spherical  cells ;  Bacterium 
forms,  short  rod-shaped  cells ;  Bacillus  forms,  longer  rod- 
shaped  cells ;  Leptothrix  forms,  simple  filaments ;  Spirillum 
forms,  spiral  filaments,  etc. 

They  multiply  by  cell  division  with  wonderful  rapidity, 
and  also  form  resting  spores  for  preservation  and  distri- 
bution. They  occur  everywhere — in  the  air,  in  the  water, 
in  the  soil,  in  the  bodies  of  plants  and  animals ;  many  of 
them  harmless,  many  of  them  useful,  many  of  them  dan- 
gerous. 

They  are  intimately  concerned  with  fermentation  and 
decay,  inducing  such  changes  as  the  souring  of  fruit  juices, 
milk,  etc.,  and  the  development  of  pus  in  wounds.  What 
is  called  antiseptic  surgery  is  the  use  of  various  means  to 
exclude  bacteria  and  so  prevent  inflammation  and  decay. 

The  pathogenic  forms— that  is,  those  which  induce  dis- 
eases of  plants  and  animals — are  of  great  importance,  and 
means  of  making  them  harmless  or  destroying  them  are 
being  searched  for  constantly.  They  are  the  causes  of  such 
diseases  as  pear-blight  and  peach-yellows  among  plants,  and 
such  human  diseases  as  tuberculosis,  cholera,  diphtheria, 
typhoid  fever,  etc. 

LICHENS 

194.  General  character. — Lichens  are  abundant  every- 
where, forming  various  colored  splotches  on  tree-trunks, 
rocks,  old  boards,  etc.,  and  growing  also  upon  the  ground 
(Figs.  269,  270,  271).  They  have  a  general  greenish-gray 
color,  but  brighter  colors  may  also  be  observed. 

The  great  interest  connected  with  Lichens  is  that  they  are 
not  single  plants,  but  each  Lichen  is  formed  of  a  fungus  and 
an  alga,  living  together  so  intimately  as  to  appear  like  a  single 


TIIALLOPIIYTES :  FUNGI 


295 


plant.    In  other  words,  a  Lichen  is  not  an  individual,  but  a 
firm  of  two  individuals  very  unlike  each  other.     This  habit 


Fig.  270.    A  common  liclien  {Physcia)  growing  on  bark,  showing  the  spreading  thallus 
and  the  numerous  dark  disks  (apothecia)  bearing  the  asci.— Goldberger. 

of  living  together  has  been  called  symhiosis^  and  the  indi- 
viduals entering  into  this  relation  are  called  symhionts. 


S^^^^^^^^^^^^^^^^^^^^ 


^V^:  ■  ,^^::- 


Fig.  271.     A  common  follose  liclien  (Parnielia)  growing,  upon  a  board,  and  showing 
apothecia.— Goldberger. 
20 


296 


PLANT   STUDIES 


If  a  Lichen  be  sectioned,  the  relation  between  the  sym- 
bionts  will  be  seen  (Fig.  272).  The  fungus  makes  the  bulk 
of  the  body  with  its  interwoven  mycelial  threads,  in  the 
meshes  of  which  lie  the  Alga3,  sometimes  scattered,  some- 


FiG.  272.    Section  through  thallus  of  a  lichen  (Siida),  showing  holdfasts  (r),  lower  (u) 
and  upper  (o)  surfaces,  fungus  hyphse  (m),  and  enmeshed  algse  (g). — After  Sachs. 


times  massed.  It  is  these  enmeshed  Algae,  showing  through 
the  transparent  mycelium,  that  give  the  greenish  tint  to 
the  Lichen. 

In  the  case  of  Lichens  the  symbionts  are  thought  by 
some  to  be  mutually  helpful,  the  alga  manufacturing  food 
for  the  fungus,  and  the  fungus  providing  protection  and 
water  containing  food  materials  for  the  alga.  Others  do  not 
recognize  any  special  benefit  to  the  alga,  and  see  in  a  Lichen 
simply  a  parasitic  fungus  living  on  the  products  of  an  alga. 
In  any  event  the  Algse  are  not  destroyed  but  seem  to  thrive. 
It  is  discovered  that  the  alga  symbiont  can  live  quite  inde- 


THALLOPHYTES:  FUNGI 


297 


pendently  of  the  fungus.  In  fact,  the  enmeshed  Algae  are 
often  recognized  as  identical  with  forms  living  independ- 
ently, those  thus  used  being  various  Blue-green,  Protococ- 
cus,  and  Conferva  forms  (see  p.  159). 

On  the  other  hand,  the  fungus  symbiont  has  become 
quite  dependent  upon  the  alga,  and  its  germinating  spores 
do  not  develop  far  unless  the  young  mycelium  can  lay  hold 
of  suitable  Algae.  At  certain  times  cup-like  or  disk-like 
bodies  appear  on  the  surface  of  the  lichen  thallus,  with 
brown,  or  black,  or  more  brightly-colored  lining  (Figs.  270, 
271).  These  bodies  are  the  apothecia.,  and  a  section  through 
them  shows  that  the  colored  lining  is  largely  made  up  of 
delicate  sacs  containing  spores  (Figs.  273,  274).  These  sacs 
are  evidently  asci,  the  apothecia  correspond  to  ascocarps, 
and  the  Lichen  fungus  proves  to  be  an  Ascomycete. 


Fig.  273.  Section  through  an  apothecinm  of  Anaptychia,  showing  stalk  of  the  cup 
(m),  masses  of  algal  cells  ig),  outer  margin  of  cup  (?•),  overlapping  edge  (t,  t),  layer 
of  asci  (A),  and  massing  of  hyphte  beneath  asci  (j^).— After  Sachs. 

Certain  Ascomycetes,  therefore,  have  learned  to  use  cer- 
tain Algae  in  this  peculiar  way,  and  a  Lichen  is  the  result. 
Some  Basidiomycetes  have  also  learned  the  same  habit,  and 
form  Lichens. 

Various  forms  of  Lichen  bodies  can  be  distinguished  as 
follows  :  (1)  Crustaceous  LicJiois^  in  which  the  thallus  resem- 


298 


PLANT   STUDIES 


bles  an  incrustation  upon  its  substratum  of  rock,  soil,  etc. ; 
(2)  Foliose  Lichens^  with  flattened,  leaf -like,  lobed  bodies,  at- 


FiG.  274.  Much  enlarged  section  of  a  portion  of  the  apothecium  of  Anaptychia,  show- 
ing the  fungus  mycelium  (m),  which  is  massed  above  (y),  just  beneath  the  layer  of 
asci  (i,  ^,  5,  li),  in  which  spores  in  various  stages  of  development  are  shown.— 
After  Sachs. 


tached  only  at  the  middle  or  irregularly  to  the  substratum ; 
(3)  Fruticose  Lichens^  with  filamentous  bodies  branching 
like  shrubs,  either  erect,  pendulous,  or  prostrate. 


I 


CHAPTER   XIX 

BRYOPHYTES  (MOSS  PLANTS) 

195.  Summary  from  Thallophytes. — Before  considering  the 
second  great  division  of  plants  it  is  well  to  recall  the  most 
important  facts  connected  with  the  Thallophytes,  those 
things  ^^hich  may  be  regarded  as  the  contribution  of  the 
Thallophytes  to  the  evolution  of  the  plant  kingdom,  and 
which  are  in  the  background  when  one  enters  the  region  of 
the  Bryophytes. 

(1)  Increasing  complexity  of  the  body. — Beginning  with 
single  isolated  cells,  the  plant  body  attains  considerable 
complexity,  in  the  form  of  simple  r.r  branching  filaments, 
cell-plates,  and  cell-masses. 

(2)  Appearance  of  spores. — The  setting  apart  of  repro- 
ductive cells,  known  as  spores,  as  distinct  from  nutritive 
cells,  and  of  reproductive  organs  to  organize  these  spores, 
represents  the  first  important  difterentiation  of  the  plant 
body  into  nutritive  and  reproductive  regions. 

(3)  Differentiation  of  spores. — After  the  introduction  of 
spores  they  become  different  in  their  mode  of  origin,  but 
not  in  their  power.  The  asexual  spore,  ordiiuirily  formed 
by  cell  division,  is  followed  by  the  appearance  of  the  sexual 
spore,  formed  by  cell  union,  the  act  of  cell  union  being 
knov/n  as  the  sexual  process. 

(4)  Differentiation  of  gametes.— At  the  first  appearance 
of  sex  the  sexual  cells  or  gametes  are  alike,  but  after- 
ward they  become  different  in  size  and  activity,  the  large 

passive  one  being  called  the  egg,  the  small  active  one  the 

299 


300  PLANT   STUDIES 

sperm,  the  organs  producing  the  two  being  known  as  oogo- 
nium and  antheridium  respectively. 

(5)  Algm  the  main  line. — The  Algae,  aquatic  in  habit, 
appear  to  be  the  Tliallophytes  which  lead  to  the  Bryophytes 
and  higher  groups,  the  Fungi  being  regarded  as  their  de- 
generate descendants  ;  and  among  the  Algae  the  Chloro- 
phyceae  seem  to  be  most  probable  ancestors  of  higher  forms. 
It  should  be  remembered  that  among  these  Green  Algae  the 
ciliated  swimming  spore  (zoospore)  is  the  characteristic 
asexual  spore,  and  the  sexual  spore  (zygote  or  oospore)  is 
the  resting  stage  of  the  plant,  to  carry  it  over  from  one 
growing  season  to  the  next. 

196.  General  characters  of  Bryophytes. — The  name  given 
to  the  group  means  '^  moss  plants,"  and  the  Mosses  may  be 
regarded  as  the  most  representative  forms.  Associated 
with  them  in  the  group,  however,  are  the  Liverworts,  and 
these  two  groups  are  plainly  distinguished  from  the  Thallo- 
phytes  below,  and  from  the  Pteridophytes  above.  Starting 
with  the  structures  that  the  Algae  have  worked  out,  the 
Bryophytes  modify  them  still  further,  and  make  their  own 
contributions  to  the  evolution  of  the  plant  kingdom,  so 
that  Bryophytes  become  much  more  complex  than  Thallo- 
phytes. 

197.  Alternation  of  generations. — Probably  the  most  im- 
portant fact  connected  with  the  Bryophytes  is  the  distinct 
alternation  of  generations  which  they  exhibit.  So  impor- 
tant is  this  fact  in  connection  with  the  development  of  the 
plant  kingdom  that  its  general  nature  must  be  clearly  under- 
stood. Probably  the  clearest  definition  may  be  obtained  by 
tracing  in  bare  outline  the  life  history  of  an  ordinary  moss. 

Beginning  with  the  asexual  spore,  which  is  not  ciliated, 
as  there  is  no  water  in  which  it  can  swim,  we  may  imagine 
that  it  has  been  carried  by  the  wind  to  some  spot  suitable 
for  its  germination.  It  develops  a  branching  filamentous 
growth  which  resembles  some  of  the  Conferva  forms  among 
the  Green  Algas  (Fig.  275).    It  is  prostrate,  and  is  a  regu- 


BRYOPHYTES 


301 


lar  thallus  body,  not  at  all  resembling  the  "moss  plant" 
of  ordinary  observation,  and  is  not  noticed  by  those  una- 
ware of  its  existence. 

Presently  one  or  more  buds  appear  on  the  sides  of  this 
alga-like  body  (Fig.  275,  h).     A  bud  develops  into  an  erect 


Fig.  275.  Protonema  of  moss :  A,  very  young  protonema,  showing  epore  {S)  which 
has  germinated  it;  B,  older  protonema,  showing  branching  habit,  remains  of 
epore  («),  rhizoids  (r),  and  buds  (6)  of  leafy  branches  (gametophoree).— After 
MuLLER  and  Thurgau. 


stalk  upon  which  are  numerous  small  leaves  (Figs.  276, 290). 
This  leafy  stalk  is  the  "  moss  plant "  of  ordinary  observa- 
tion, and  it  will  be  noticed  that  it  is  simply  an  erect  leafy 
branch  from  the  prostrate  alga-like  body. 

At  the  top  of  this  leafy  branch  sex-organs  appear,  cor- 
responding to  the  antheridia  and  oogonia  of  tlie  Algie,  and 
within  them  there  are  sperms  and  eggs.  A  sperm  and  Q^g 
fuse  and  an  oospore  is  formed  at  the  summit  of  the  leafy 
branch. 

The  oospore  is  not  a  resting  spore,  but  germinates  im- 
mediately, forming  a  structure  entirely  unlike  the  moss 


302 


PLANT   STUDIES 


,rh 


Fig.  276.  A  common  moss 
(Polytrichum  camryiime), 
showing  the  leafy  gameto- 
phore  with  rhizoids  (rh), 
and  two  sporophytes  (sporo- 
gonia),  with  seta  (s),  calyp- 
tra  (c),  and  opercuhim  (d), 
the  calyptra  having  been  re- 
moved.—After  SCHENCK. 


plant  from  which  it  came.  This  new 
leafless  body  consists  of  a  slender 
stalk  bearing  at  its  summit  an  urn- 
like case  in  which  are  developed  nu- 
merous asexual  spores  (Figs,  276, 292), 
This  whole  structure  is  often  called 
the  ''spore  fruit,"  and  its  stalk  is 
imbedded  at  base  in  the  summit  of 
the  leafy  branch,  thus  obtaining  firm 
anchorage  and  absorbing  what  nour- 
ishment it  needs,  but  no  more  a  part 
of  the  leafy  branch  than  is  a  para- 
site a  part  of  the  host. 

When  the  asexual  spores,  pro- 
duced by  the  ''  spore  fruit,"  germi- 
nate, they  reproduce  the  alga-like 
body  with  which  we  began,  and  the 
life  cycle  is  completed. 

In  examining  this  life  history,  it 
is  apparent  that  each  spore  produces 
a  different  structure.  The  asexual 
spore  produces  the  alga-like  body 
with  its  erect  leafy  branch,  while 
the  oospore  produces  the  ''  spore 
fruit"  with  its  leafless  stalk  and 
spore  case.  These  two  structures, 
one  produced  by  the  asexual  spore, 
the  other  by  the  oospore,  appear  in 
alternating  succession,  and  this  is 
what  is  meant  by  aUeriiation  of  gen- 
erations. 

These  two  ''generations"  differ 
strikingly  from  one  another  in  the 
spores  which  they  produce.  The 
generation  composed  of  alga -like 
body   and    erect   leafy  branch   pro- 


BRYOPHYTES  303 

duces  only  sexual  spores  (oospores),  and  therefore  produces 
sex  organs  and  gametes.  It  is  known,  therefore,  as  the 
gametophyte — that  is,  "  the  gamete  plant." 

The  generation  which  consists  of  the  "spore  fruit" — 
that  is,  leafless  stalk  and  spore  case — produces  only  asexual 
spores,  and  is  called  the  sporophyte — that  is,  "  the  spore 
plant." 

The  relation  between  the  two  alternating  generations 
may  be  indicated  clearly  by  the  following  formula,  in 
which  G  and  S  are  used  for  gametophyte  and  sporophyte 
respectively : 

G=8>o— S-0— G=8>o— S— 0— G,  etc. 

The  formula  indicates  that  the  gametophyte  produces 
two  gametes  (sperm  and  Qgg)^  which  fuse  to  form  an  oospore, 
which  produces  the  sporophyte,  which  produces  an  asexual 
spore,  which  produces  a  gametophyte,  etc. 

In  reference  to  the  sporophytes  and  gametophytes  of 
Bryophytes  two  peculiarities  may  be  mentioned  at  this 
point:  (1)  the  sporophyte  is  dependent  upon  the  gameto- 
phyte for  its  nourishment,  and  remains  attached  to  it ;  (2) 
the  gametophyte  is  the  special  chlorophyll-generation,  and 
hence  is  the  more  conspicuous. 

If  the  ordinary  terms  in  reference  to  Mosses  be  fitted 
to  the  facts  given  above,  it  is  evident  that  the  "  moss 
plant "  is  the  leafy  branch  of  the  gametophyte ;  that  the 
*'  moss  fruit "  is  the  sporophyte ;  and  that  tlie  alga-like 
part  of  the  gametophyte  has  escaped  attention  and  a 
common  name. 

The  names  now  given  to  the  different  structures  which 
appear  in  this  life  history  are  as  follows  :  The  alga-like  part 
of  the  gametophyte  is  the  protonema^  the  leafy  branch  is 
the  ^«7;?e/o;;/?ore  ("gamete-bearer  ") ;  the  whole  sporophyte 
is  the  sporogonium  (a  name  given  to  this  peculiar  leafless 
sporophyte  of  Bryophytes),  the  stalk-like  portion  is  the  seta^ 
the  part  imbedded  in  the  gametopnore  is  the  foot,  and  the 
urn-like  spore-case  is  the  capsule. 


304 


PLANT   STUDIES 


198.  The  antheridium. — The  male  organ  of  the  Bryophytes 
is  called  an  antheridium,  just  as  among  Thallophytes,  but 
it  has  a  very  different  structure.     In  general  among  the 


Fig.  277.  Sex  organs  of  a  common  moss  (Funaria):  the  group  to  the  right  represents 
an  antheridium  (A)  discharging  from  its  apex  a  mass  of  sperm  mother  cells  (a),  a 
single  mother  cell  with  its  sperm  (6),  and  a  single  sperm  (c),  showing  body  and 
two  cilia;  the  group  to  the  left  represents  an  archegonial  cluster  at  summit  of 
stem  (A),  showing  archegonia  (a),  and  paraphyses  and  leaf  sections  (b),  and  also  a 
single  archegonium  (B),  with  venter  (b)  containing  egg  and  ventral  canal  cell,  and 
neck  (h)  containing  the  disorganizing  axial  row  (neck  canal  cells).— After  Sachs. 

Thallophytes  it  is  a  single  cell  (mother  cell),  and  may  be 
called  a  simple  antheridium,  but  in  the  Bryophytes  it  is  a 
many-celled  organ,  and  may  be  regarded  as  a  compound 
antheridium.    It  is  usually  a  stalked,  club-shaped,  or  oval  to 


BRYOPHYTES 


305 


globular  body   (Figs.  277,  278).     A  section  through  this 

body  shows  it  to  consist  of  a  single  layer  of  cells,  which 

forms  the  wall  of  the  antheridium,  and 

within  this   a   compact  mass  of  small 

cubical  (square  in  section)  cells,  within 

each  one  of  which  there  is  formed  a 

single  sperm  (Fig.  278).     The  sperm  is 

a  very  small  cell  w4th  two  long  cilia 

(Fig.  277).    These  small  biciliate  sperms 

are   one  of  the  distinguishing   marks 

of  the  Bryophytes.    When  the  mature 

antheridia  are  wet  they  are  opened  at 

the  apex  and  discharge  their  contents 

(Fig.  277),  and   the   sperms   escaping 

swim  actively  about. 

199.  The  archegonium. — This  name 
is  given  to  the  female  sex  organ,  which 
is  a  many-celled  structure,  shaped  like 
a  flask  (Figs.  277,  287).  The  neck  of 
the  flask  is  more  or  less  elongated,  and 
within  the  bulbous  base  {venter)  the  single  egg  is  organized. 

To  this  neck  the  swimming  sperms  are  attracted,  enter 
an3'  pass  doAvn  it,  one  of  them  fuses  with  the  egg^  and  this 
act  of  fertilization  results  in  an  oospore. 

200.  Germination  of  the  oospore. — The  oospore  in  Bryo- 
phytes is  not  a  resting  spore,  but  germinates  immediately 
by  cell  division,  forming  the  sporophyte  embryo,  which 
presently  develops  into  the  mature  sporophyte  (Fig.  279,  ^4). 
The  lower  part  of  the  embryo  develops  the  foot,  which 
obtains  a  Arm  anchorage  in  the  gametophore  by  the  latter 
growing  up  around  it  (Fig.  279,  B^  C).  The  upper  part  of 
the  embryo  develops  upward,  organizing  the  seta  and  cap- 
sule. As  the  embryo  increases  in  size,  the  venter  of  the 
archegonium  grows  also,  forming  what  is  called  the  caJj/pfra-, 
and  in  true  mosses  the  embryo  presently  breaks  loose  the 
calyptra  at  its  base  and  carries  it  upward  perched  on  the  top 


Fig.  278.  Antheridium  of 
a  liverwort  in  section, 
showing  single  layer 
of  wall  cells  surround- 
ing the  mass  of  moth- 
er cells.— After  Stras- 

BURGER. 


306 


PLANT  STUDIES 


of  the  capsule  like   a  loose   cap  or  hood   (Fig.   276,  c), 
which   sooner   or  later  falls   off.     As   stated    before,   the 

mature  structure 
developed  from 
the  oospore  or  egg  is 
called  a  sporogoni- 
um,  a  form  of  sporo- 
phyte  peculiar  to  the 
Bryophytes. 

201.  The  sporogoni- 
um. — In  its  fullest  de- 
velopment the  sporogo- 
nium  is  differentiated 
into  the  three  regions, 
foot,  seta,  and  capsule 
(Fig.  276) ;  but  in  some 
forms  the  seta  may  be 
lacking,  and  in  others 
the  foot  also,  the  sporo- 
gonium  in  this  last 
case  being  only  the 
capsule  or  spore  case, 
which,  after  all,  is  the 
essential  part  of  any 
sporogonium. 

At  first  the  capsule 
is  solid,  and  its  cells 
are  all  alike.  Later  a 
group  of  cells  within 
begins  to  differ  in  ap- 
pearance from  those 
about  them,  being  set 
apart  for  the  produc- 
tion of  spores.  This 
initial  group  of  spore-producing  cells  is  called  the  arcJie- 
sporium,  a  word  meaning  "  the  beginning  of  spores." 


Fig.  279.  Sporogonium  of  Funaria  :  A,  an  em- 
bryo sporogonium  (/,/'),  developing  within 
the  venter  (b,  b)  of  an  archegonium ;  B,  C, 
tips  of  leafy  shoots  bearing  young  sporo- 
gonia,  pushing  up  calyptra  (c)  and  archego- 
nium neck  (h).  and  the  foot  becoming  im- 
bedded in  the  apex  of  the  gametophore. — 
After  GoEBEL, 


BRYOPIIYTES  307 

The  archesporium  forms  new  cells,  and  the  last  ones 
formed  are  mother  cells,  in  each  one  of  which  four  spores 
are  organized,  the  group  of  four  being  called  a  tetrad. 
Among  Bryophytes  and  the  higher  groups  asexual  spores 
are  always  produced  in  tetrads. 

After  the  spores  are  formed  the  walls  of  the  mother 
cells  disorganize,  and  the  spores  are  left  lying  loose  in 
a  cavity  which  was  formerly  occupied  by  the  sporogenous 
tissue.  All  mother  cells  do  not  always  organize  spores. 
In  some  cases  some  of  them  are  used  up  in  supplying  nour- 
ishment to  those  which  form  spores.  In  other  cases,  certain 
mother  cells  become  much  modified  in  form,  being  organ- 
ized into  elongated,  spirally-banded  cells  called  elaters  (Fig. 
286),  meaning  "drivers"  or  "hurlers."  These  elaters  lie 
among  the  loose  ripe  spores,  are  discharged  with  them,  and 
by  their  jerking  movements  assist  in  scattering  them. 

The  sporogonium  is  a  very  important  structure  from 
the  standpoint  of  evolution,  for  it  represents  the  conspicu- 
ous part  of  the  higher  plants.  The  "fern  plant,"  and 
the  herbs,  shrubs,  and  trees  among  "  flowering  plants," 
correspond  to  the  sporogonium  of  Bryophytes,  and  not  to 
the  leafy  branch  (gametophope)  or  "  moss  plant." 


CHAPTEE  XX 

THE  GREAT  GBOUPS  OF  BRYOPHYTES 

HepatiojE  (Liverworts) 

202.  General  character. — Liverworts  live  in  a  variety  of 
conditions,  some  floating  on  the  water,  many  in  damp 
places,  and  many  on  the  bark  of  trees.  In  general  they  are 
moisture-loving  plants  (hydrophytes),  though  some  can  en- 
dure great  dryness.  The  gametophyte  body  is  prostrate, 
though  there  may  be  erect  and  leafless  gametophores. 

This  prostrate  habit  develops  a  dorsiventral  body — that 
is,  one  whose  two  surfaces  {dorsal  and  ventral)  are  exposed 
to  different  conditions  and  become  unlike  in  structure.  In 
Liverworts  the  ventral  surface  is  against  the  substratum, 
and  puts  out  hair-like  processes  (rhizoids)  for  anchorage 
and  possibly  absorption.  The  dorsal  region  is  exposed  to 
the  light  and  its  cells  develop  chlorophyll.  If  the  thallus 
is  thin,  chlorophyll  is  developed  in  all  the  cells ;  if  it  be  so 
thick  that  the  light  is  cut  off  from  the  ventral  cells,  the 
thallus  is  differentiated  into  a  green  dorsal  region  doing 
the  chlorophyll  work,  and  a  colorless  ventral  region  pro- 
ducing anchoring  rhizoids.  This  latter  represents  a  simple 
differentiation  of  the  nutritive  body  into  working  regions, 
the  ventral  region  absorbing  material  and  conducting  it  to 
the  green  dorsal  cells  which  use  it  in  making  food. 

There  seem  to  have  been  at  least  three  main  lines  of 
development  among  Liverworts,  each  beginning  in  forms 
with  a  very  simple  thallus,  and   developing   in   different 
directions.     They  are  briefly  indicated  as  follows : 
308 


THE  GREAT  GROUPS  OF  BRYOPHYTES 


309 


203.  Marchantia  forms. — In  this  line  the  simple  thallus 
gradually  becomes  changed  into  a  very  complex  one.  The 
thallus  retains  its  simple 
outlines,  but  becomes  thick 
and  differentiated  in  tissues 
(groups  of  similar  cells). 
The  line  may  be  distin- 
guished, therefore,  as  one 
in  which  the  differentia- 
tion of  the  tissues  of  the 
gametophyte  is  emphasized 
(Figs.  280-282).  In  3far- 
chantia  proper  the  thallus 
becomes  very  complex,  and 
it  may  be  taken  as  an  illus- 
tration. 

The  thallus  is  so  thick 
that  there  are  very  distinct 
green   dorsal    and    colorless 

ventral  regions  (Fig.  283).  The  latter  puts  out  numerous 
rhizoids  and  scales  from  the  single  layer  of  epidermal  cells. 
Above  the  ventral  epidermis  are  several  layers  of  colorless 


Fig.  280.  A  very  small  species  of  Biccia, 
one  of  the  Marchantia  forms  :  A,  a 
group  of  thallus  bodies  slightly  en- 
larged ;  B,  section  of  a  thallus,  show- 
ing rhizoids  and  two  sporogonia  im- 
bedded and  communicating  with  the 
outside  by  tubular  passages  in  the 
thallus.— After  Stkasburger. 


Fio.  281.  liicciocai'pus,  a  Marchantia  form,  showing  numerous  rhizoids  from  ventral 
surface,  the  dichotomous  branching,  and  the  position  of  the  sporogonia  on  the 
dorsal  surface  along  the  "  midribs."— Goldberger. 


Fig.  282.  Two  common  liverworts  :  to  the  left  is  Conocephalus,  a  Marchantia  form, 
showing  rhizoids,  dichotomous  branching,  and  the  conspicuous  rhombic  areas 
(areolae)  on  the  dorsal  surface;  to  the  right  is  Anthoceros,  with  its  simple  thallus 
and  pod-like  sporogonia.— Goldberger. 


Fig. 283.  Cross-sections  of  thallus  of  Marchantia:  A,  section  from  thicker  part  of 
thallus,  where  supporting  tissue  (p)  is  abundant,  and  showing  lower  epidermis 
giving  rise  to  rhizoids  {h)  and  plates  (&),  also  chlorophyll  tissue  ichl)  organized 
into  chambers  by  partitions  (o)\  B,  section  near  margin  of  thallus  more  magnified, 
showing  lower  epidermis,  two  layers  of  supporting  tissue  {p)  with  reticulate  walls, 
a  single  chlorophyll  chamber  with  its  bounding  walls  {s)  and  containing  short, 
often  brandling  filaments  whose  cells  contain  chloroplasts  {cJil),  overarching 
upper  epidermis  (o)  pierced  by  a  large  chimney-like  air-pore  (sj»).— After  Goebel. 


Fig.  284.  Section  through  cnpule  of  Marchantia,  showing  wall  in  which  are  chloro- 
phyll-bearing air-chambers  with  air-pores,  and  gemmae  {a)  in  various  stages  of 
development.— After  Dodel-Port. 


Fig.  2R5.  Marchantia  pnlymnrphn  :  the  lower  figure  represents  a  pametophyte  bear- 
ing a  mature  antheridial  branch  (d),  some  young  antheridial  branches,  and  also 
some  cupules  with  toothed  margins,  in  which  the  gemma;  may  be  seen  ;  the 
upper  figure  represents  a  partial  section  through  the  antheridial  disk,  and  shows 
antheridia  within  the  antheridial  cavities  (a,  b,  c,  d,  «,/).— After  Kny. 
21 


312 


PLANT   STUDIES 


cells  more  or  less  modified  for  conduction.  Above  these 
the  dorsal  region  is  organized  into  a  series  of  large  air  cham- 
bers, into  which  project  chlorophyll-containing  cells  in  the 


Fig.  286.  Marchantia  polymorpha,  a  common  liverwort :  i,  thallus,  with  rhizoids, 
bearing  a  mature  archegonial  branch  (/)  and  several  younger  ones  (a,  b,  c,  d,  €)\ 
2  and  3,  dorsal  and  ventral  views  of  archegonial  disk;  h  and  5,  young  eporophyte 
(eporogonium)  embryos;  6,  more  mature  sporogonium  still  within  enlarged  venter 
of  archegonium;  7,  mature  sporogonium  discharging  spores;  8,  three  spores  and 
an  elater.— After  Kny. 


form  of  short  branching  filaments.  Overarching  the  air 
chambers  is  the  dorsal  epidermis,  and  piercing  through  it 
into  each  air  chamber  is  a  conspicuous  air  pore  (Fig.  283,  B). 


THE   GREAT   GROUPS   OF   BRYOl'liYTES 


313 


The  air  chambers  are  outlined  on  the   surface  as   small 

rhombic  areas  {areolcs),  each  containing  a  single  air  pore. 

Peculiar  reproductive  bodies  are  also   developed  upon 

the  dorsal  surface  of  Marcliantia  for  vegetative  multiplica- 


Fig.  287.  Marchantia  polymorpha :  1,  partial  section  through  archegonial  diek,  show- 
ing archt'gonia  with  long  necks,  and  venters  containing  eggs;  9,  young  archego- 
ninm  showing  axial  row;  10,  superficial  view  at  later  stage;  11,  mature  archego- 
nium,  with  axial  row  disorganized  and  leaving  an  open  passage  to  the  large  egg; 
12,  cross-section  of  venter;  13,  cross-section  of  neck.— After  Knt. 


tion.  Little  cups  (cupules)  appear,  and  in  them  are  numer- 
ous short-stalked  bodies  {gemmxe)^  which  are  round  and 
flat  (biscuit-shaped)  and  many-celled  (Figs.  284,  285).     The 


314 


PLANT   STUDIES 


gemmae  fall  off  and  develop  new  thallus  bodies,  making 
rapid  multiplication  possible.  Marchantia  also  possess  re- 
markably prominent  gametophores,  or  "  sexual  branches " 
as  they  are  often  called.  In  this  case  the  gametophores  are 
differentiated,  one  bearing  only  antheridia  (Fig.  285),  and 
known  as  the  "  antheridial  branch,"  the  other  bearing  only 
archegonia  (Figs.  286,  287),  and  known  as  the  "  archegonial 
branch."  The  scalloped  antheridial  disk  and  the  star- 
shaped  archegonial  disk,  each  borne  up  by  the  stalk-like 
gametophore,  are  seen  in  the  illustrations. 

204.  Jungermannia  forms. — This  is  the  greatest  line  of 
the  Liverworts,  the  forms  being  much  more  numerous  than 
in  the  other  lines.     They  grow  in  damp  places ;  or  in  drier 


Fig.  288.  Two  liverworts,  both  Jungermannia  forms:  to  the  left  is  Blasia,  which 
retains  the  thallus  forms  but  has  lobed  margins  ;  to  the  right  is  Scapania,  with 
distinct  leaves  and  sporogonia  (^4).— Goldberger. 

situations  on  rocks,  ground,  or  tree-trunks  ;  or  in  the  tropics 
also  on  the  leaves  of  forest  plants.  They  are  generally  deli- 
cate jDlants,  and  resemble  small  Mosses,  many  of  them  doubt- 
less being  commonly  mistaken  for  Mosses  (Fig.  288). 

In  this  line   the  thallus  gradually  passes   into   bodies 


THE  GREAT  GROCPS  OF  BKYOPIIYTES 


315 


organized  into  a  central  stem-like  axis  bearing  two  rows  of 
small,  often  crowded  leaves.  In  consequence  of  this  such 
Ju  ngermannia  forms 
are  usually  called  "  leafy 
liverworts,"  to  distin- 
guish them  from  the 
other  Liverworts,  which 
are  "  thallose."  They  are 
also  often  called  "scale 
mosses,"  on  account  of 
their  moss-like  appear- 
ance and  their  small 
scale-like  leaves. 

205.  Anthoceros  forms. 
— This  line  contains  com- 
paratively few  forms,  but 
they  are  of  great  interest, 
as  they  are  supposed  to 
represent  forms  which 
have  given  rise  to  the 
Mosses,  and  possibly  to 
the  Pteridophytes  also. 
The  thallus  is  very  sim- 
ple, being  differentiated 
neither  in  structure  nor 
form,  as  in  the  two  other 
lines  ;  but  the  special  de- 
velopment has  been  in 
connection  with  the  spo- 
rogonium  (Figs.  282,  289).  This  complex  sporogonium 
(sporophyte)  has  a  large  bulbous  foot  imbedded  in  the 
simple  thallus,  while  above  there  arises  a  long  pod-like 
capsule. 

The  chief  direction  of  the  development  of  tlie  three  liv- 
erwort lines  may  be  summed  up  briefly  as  follows  :  The 
Marcliantia  line   has  differentiated   the   structure  of   the 


Fig.   289, 


Anthoceros  gracUis :  A,  several 
gametophytes,  on  which  sporogonia  have 
developed  ;  B,  an  enhxrged  sporogonium, 
showing  its  elongated  character  and  de- 
hiscence by  two  valves  leaving  exposed 
the  slender  columella  on  the  surface  of 
which  are  the  spores  ;  C,  D,  E,  F,  ela- 
ters  of  various  forms  ;    G,  spores.— After 

SCHIFFNER. 


316  PLANT   STUDIES 

garnet ophyte ;  the  Jungermannia  line  has  differentiated 
the  form  of  the  gametophyte ;  the  Anthoceros  line  has 
differentiated  the  structure  of  the  sporophyte.  It  should 
be  remembered  that  other  characters  also  serve  to  distin- 
guish the  lines  from  one  another. 

Musci  (Mosses) 

206.  General  character. — Mosses  are  highly  specialized 
plants,  probably  derived  from  Liverworts,  the  numerous 
forms  being  adapted  to  all  conditions,  from  submerged  to 
very  dry,  being  most  abundantly  displayed  in  temperate 
and  arctic  regions.  Many  of  them  may  be  dried  out  com- 
pletely and  then  revived  in  the  presence  of  moisture,  as  is 
true  of  many  Lichens  and  Liverworts,  with  which  forms 
Mosses  are  very  commonly  associated. 

They  also  have  great  power  of  vegetative  multiplica- 
tion, new  leafy  shoots  putting  out  from  old  ones  and  from 
the  protonema  indefinitely,  thus  forming  thick  carpets  and 
masses.  Bog  mosses  often  completely  fill  up  bogs  or  small 
ponds  and  lakes  with  a  dense  growth,  which  dies  below 
and  continues  to  grow  above  as  long  as  the  conditions  are 
favorable.  These  quaking  bogs  or  "mosses,"  as  they  are 
sometimes  called,  furnish  very  treacherous  footing  unless 
rendered  firmer  by  other  plants.  In  these  moss-filled  bogs 
the  water  shuts  off  the  lower  strata  of  moss  from  complete 
disorganization,  and  they  become  modified  into  a  coaly 
substance  called  peat,  which  may  accumulate  to  consid- 
erable thickness  by  the  continued  upward  growth  of  the 
mass  of  moss. 

The  gametophyte  body  is  differentiated  into  two  very 
distinct  regions  :  (1)  the  prostrate  dorsi ventral  thallus, 
which  is  called  protonema  in  this  group,  and  which  may 
be  either  a  broad  flat  thallus  or  a  set  of  branching  fila- 
ments (Figs.  275,  290)  ;  (2)  the  erect  leafy  branch  or 
gametophore  (Fig.  276).     This  erect  branch  is  said  to  be 


THE  GREAT  GROUPS  OF  BRYOPHYTES 


317 


radial^  in  contrast  with  the  dorsiventral  thallus,  referring 
to  the  fact  that  it  is  exposed  to  similar  conditions  all 
around,  and  its  organs  are  arranged  about  a  central  axis 
like  the  parts  of  a  radiate  animal.  This  position  is  much 
more  favorable  for  the 
chlorophyll  work  than 
the  dorsiventral  posi- 
tion, as  the  special 
chlorophyll  organs 
(leaves)  can  be  spread 
out  to  the  light  freely 
in  all  directions. 

The  leafy  branch 
of  the  Mosses  usually 
becomes  independent 
of  the  thallus  by  put- 
ting out  rhizoids  at  its 
base  (Fig.  290),  the 
thallus  part  dying. 
Sometimes,  however, 
the  filamentous  proto- 
nema  is  very  persist- 
ent, and  gives  rise  to  a 
perennial  succession  of 
leafy  branches. 

At  the  summit  of 
the  leafy  gametophore, 
either  upon  the  main 
axis  or  upon  a  lateral 
branch,  the  antheridia  and  archegonia  are  borne  (Fig.  277). 
Often  the  leaves  at  the  summit  become  modified  in  form 
and  arranged  to  form  a  rosette,  in  the  center  of  which 
are  the  sex  organs.  This  rosette  is  often  called  the  "  moss 
fiower,"  but  it  holds  no  relation  to  the  flower  of  Seed- 
plants,  and  the  phrase  should  not  be  used.  A  rosette  may 
contain  but  one  kind  of  sex  organ  (Fig.  277),  or  it  may 


Fig.  290.  A  moss  (Bryu7?i),  showing  base  of  a 
leafy  branch  (gametophore)  attached  to  the 
protonema,  and  having  sent  out  rhizoids.  On 
the  protonemal  fihiment  to  the  right  and  be- 
low is  the  young  bud  of  another  leafy  branch, 

— MULLER. 


318 


PLANT   STUDIES 


contain  both  kinds,  for  Mosses  are  both  dioecious  and  monoe- 
cious.    The  two  principal  groups  are  as  follows  : 

207.  Sphagnum  forms. — These  are  large  and  pallid  bog 
mosses,  found  abundantly  in  marshy  ground,  especially  of 
temperate  and  arctic  regions,  and  are  conspicuous  peat- 


B  C  D     \l    E 

Pig.  291.  Sphagnum:  J.,  a  leafy  branch  (gametophore)  bearing  four  mature  sporo' 
gonia  ;  B,  archegonium  in  whose  venter  a  young  embryo  sporophyte  (em)  is  de- 
veloping ;  C,  section  of  a  young  sporogonium  (sporophyte),  showing  the  bulbous 
foot  {spf)  imbedded  in  the  apex  of  the  pseudopodium  (ps),  the  capsule  (k),  the 
columella  ico)  capped  by  the  dome-shaped  archesporium  (s]m),  a  portion  of  the 
calyptra  ica),  and  the  old  archegonium  neck  {ah)  ;  Z>,  branch  bearing  mature 
sporogonium  and  showing  pseudopodium  (j^s),  capsule  (k),  and  operculum  (d) ; 
E,  antheridium  discharging  sperms  ;  F,  a  single  sperm,  showing  coiled  body  and 
two  cilia.— After  Schimper. 


formers  (Fig.  291).  The  leaves  and  gametophore  axis  are 
of  peculiar  structure  to  enable  them  to  suck  up  and  hold  a 
large  amount  of  water.  This  abundant  water-storage  tissue 
and  the  comparatively  poor  display  of  chlorophyll-contain- 
ing cells  gives  the  peculiar  pallid  appearance. 

208.  True  Mosses. — This  immense  and  most  highly  organ- 
ized Bryophyte  group  contains  the  great  majority  of  the 


THE  GKEAT  GROUPS  OF  BRYOPHYTES 


319 


Mosses,  which  are  sometimes  called  the  Bryuni  forms,  to 
distinguish  them  from  the  Spliagnum  forms.  They  are 
the  representative  Bryophytes,  the  only  group  vying  with 
them  being  the  leafy 
Liverworts,  or  Junger- 
mannia  forms.  They 
grow  in  all  conditions 
of  moisture,  from  actual 
submergence  in  water  to 
dry  rocks,  and  they  also 
form  extensive  peat  de- 
posits in  bogs. 

The  sporogonium  has 
a  foot  and  usually  a  long 
slender  seta,  but  the  cap- 
sule is  especially  com- 
plex. When  the  lid-like 
operculum  falls  off,  the  capsule  is  left  like  an  urn  full  of 
spores,  and  at  the  mouth  of  the  urn  there  is  usually  dis- 
played a  set  of  slender,  often  very  beautiful  teeth  (Fig. 
292),  converging  from  the  circumference  toward  the  center, 
and  called  the  j^eristome^  meaning  "about  the  mouth." 
These  teeth  by  bending  inward  and  outward  help  to  dis- 
charge the  spores. 


Fig.  292.  Sporogonia  of  Grimmia,  from  all  of 
which  the  operculum  has  fallen,  displaying 
the  peristome  teeth:  A,  position  of  the  teeth 
when  dry  ;  B,  position  when  moist.— After 
Kerneu. 


CHAPTEE  XXI 

PTERIDOPHYTES  (FERN  PLANTS) 

)^09.  Summary  from  Bryophytes. — In  introducing  the  Bryo- 
phytes  a  summary  from  the  Thallophytes  was  given  (see  § 
60),  indicating  certain  important  things  which  that  group 
has  contributed  to  the  evolution  of  the  plant  kingdom. 
In  introducing  the  Pteridophytes  it  is  well  to  notice  certain 
important  additions  made  by  the  Bryophytes. 

(1)  Alternation  of  generations. — The  great  fact  of  alter- 
nating sexual  (gametophyte)  and  sexless  (sporophyte)  gen- 
erations is  first  clearly  expressed  by  the  Bryophytes,  although 
its  beginnings  are  to  be  found  among  the  Thallophytes. 
Each  generation  produces  one  kind  of  spore,  from  which  is 
developed  the  other  generation. 

(2)  Gametophyte  the  chlorophyll ge7ieration. — On  account 
of  this  fact  the  food  is  chiefly  manufactured  by  the  gameto- 
phyte, which  is  therefore  the  more  conspicuous  generation. 
When  a  moss  or  a  liverwort  is  spoken  of,  therefore,  the 
gametophyte  is  usually  referred  to. 

(3)  Gametophyte  and  sporophyte  7iot  independent. — The 
sporophyte  is  mainly  dependent  upon  the  gametophyte  for 
its  nutrition,  and  remains  attached  to  it,  being  commonly 
called  the  sporogonium,  and  its  only  function  is  to  produce 
spores. 

(4)  Differentiation  of  thallus  i^ito  stem  and  leaves. — 
This  appears  incompletely  in  the  leafy  Liverworts  {Junger- 
mannia  forms)  and  much  more  clearly  in  the  erect  and 
radial  leafy  branch  (gametophore)  of  the  Mosses. 

320 


PTEKIDOPHVTES 


321 


(5)  Many-ceJhd  sex  organs. — The  antheridia  and  the 
flask-shaped  archegonia  are  very  characteristic  of  Bryo- 
phytes  as  contrasted  with  Thallophytes. 

210.  General  characters  of  Pteridophjrtes. — The  name  means 
*'fern  phmts/''  and  the  Ferns  are  the  most  numerous  and  the 
most  representative  forms  of  the  group.  Associated  with 
them,  however,  are  the  Horsetails  (Scouring  rushes)  and 
the  Club-mosses.  By  many  the  Pteridophytes  are  thought 
to  have  been  derived  from  such  Liverworts  as  the  xintlio- 
ceros  forms,  while  some  think  that  they  may  possibly  have 
been  derived  directly  from  the  Green  Algge.  AVhatever 
their  origin,  they  are  very  distinct  from  Bryophytes. 

One  of  the  very  important  facts  is  the  appearance  of 
the  vascular  system,  which  means  a  *' system  of  vessels," 
organized  for  conducting  material  through  the  plant  body. 
The  appearance  of  this  system  marks  some  such  epoch  in 
the  evolution  of  plants  as  is  marked  in  animals  by  the 
appearance  of  the  "backbone."  As  animals  are  often 
grouped  as  "vertebrates"  and  "invertebrates,"  plants  are 
often  grouped  as  "vascular  plants"  and  "non-vascular 
plants,"  the  former  being  the  Pteridophytes  and  Spermato- 
phytes,  the  latter  being  the  Thallophytes  and  Bryophytes. 
Pteridophytes  are  of  great  interest,  therefore,  as  being  the 
first  vascular  plants. 

211.  Alternation  of  generations. — This  alternation  con- 
tinues in  the  Pteridophytes,  but  is  even  more  distinct  than 
in  the  Bryophytes,  the  gametophyte  and  sporophyte  be- 
coming independent  of  one  another.  An  outline  of  the  life 
history  of  an  ordinary  fern  will  illustrate  this  fact,  and  will 
serve  also  to  point  out  the  prominent  structures.  Upon  the 
lower  surface  of  the  leaves  of  an  ordinary  fern  dark  spots 
or  lines  are  often  seen.  These  are  found  to  yield  spores, 
with  which  the  life  history  may  be  begun. 

When  such  a  spore  germinates  it  gives  rise  to  a  small, 
green,  heart-shaped  thallus,  resembling  a  delicate  and  sim- 
ple liverwort  (Fig.  293,  A).     Upon  this  thallus  antheridia 


322 


PLANT   STUDIES 


and  archegonia  appear,  so  that  it  is  evidently  a  gameto- 
pliyte.  This  gametophyte  escapes  ordinary  attention,  as  it 
is  usually  very  small,  and  lies  prostrate  upon  the  substra- 
tum. It  has  received  the  name  prothallium  or  jji^othallus, 
so  that  when  the  term  prothallium  is  used  the  gametophyte 
of  Pteridophytes  is  generally  referred  to  ;  j  ust  as  when  the 
term  sporogonium  is  used  the  sporophyte  of  the  Bryophytes 
is  referred  to.  Within  an  archegonium  borne  upon  this  little 
prothallium  an  oospore  is  formed.     When  the  oospore  ger- 


FiG.  293.  Prothallium  of  a  common  fern  (Aspidium):  A,  ventral  surface,  showing 
rhizoids  {rh),  antheridia  {an),  and  archegonia  {ar) ;  B,  ventral  surface  of  an  older 
gametophyte,  showing  rhizoids  (rh)  and  young  sporophyte  with  root  {w)  and  leaf 
(6).— After  ScHENCK. 

minates  it  develops  the  large  leafy  plant  ordinarily  spoken 
of  as  ^'the  fern,"  with  its  subterranean  stem,  from  which 
roots  descend,  and  from  which  large  branching  leaves  rise 
above  the  surface  of  the  ground  (Fig.  293,  B).  It  is  in 
this  complex  body  that  the  vascular  system  appears.  Xo 
sex  organs  are  developed  upon  it,  but  the  leaves  bear  numer- 
ous sporangia  full  of  asexual  spores.  This  complex  vascular 
plant,  therefore,  is  a  sporophyte,  and  corresponds  in  this 
life  history  to  the.  sporogonium  of  the  Bryophytes.     This 


PTEKIDOPHYTES 


323 


completes  the  life  cycle,  as  the  asexual  spores  develop  the 
prothallium  again. 

In  contrasting  this  life  history  with  that  of  Bryophytes 
several  important  differences  are  discovered.  The  most, 
striking  one  is  that  the  sporophyte  has  become  a  large, 
leafy,  vascular,  and  independent  structure,  not  at  all  re- 
sembling its  representative  (the  sporogonium)  among  the 
Bryophytes. 

Also  the  gametophyte  has  become  much  reduced,  as 
compared  with  the  gametophytes  of  the  larger  Liverworts 
and  Mosses.  It  seems  to  have  resumed  the  simplest  liver- 
wort form. 

212.  The  gametophyte. — The  prothallium,  like  a  simple 
liverwort,  is  a  dorsiventral  body,  and  puts  out  numerous 


Pig.  294.  Archegonium  of  Ptens  at  the  time  of  fertilization,  showing  tissue  of  gam- 
etophyte {A),  the  cells  forming  the  neck  (iJ),  the  passageway  formed  by  the  dis- 
organization of  the  canal  cells  (C),  and  the  egg  (Z>)  lying  exposed  in  the  venter. 
—Caldwell. 


rhizoids  from  its  ventral  surface  (Fig.  293).  It  is  so  thin 
that  all  the  cells  contain  chlorophyll,  and  it  is  usually  short- 
lived. 


324 


PLANT   STUDIES 


At  the  bottom  of  the  conspicuous  notch  in  the  prothal- 
lium  is  the  growing  point,  representing  the  apex  of  the 
plant.     This  notch  is  always  a  conspicuous  feature. 

The  antheridia  and  archegonia  are  usually  developed  on 
the  under  surface  of  the  prothallium  (Fig.  293,  A)^  and  dif- 
fer from  those  of  all  Bryophytes,  except  the  Antlioceros 
forms,  in  being  sunk  in  the  tissue  of  the  prothallium  and 
opening  on  the  surface,  more  or  less  of  the  neck  of  the 
archegonium  projecting  (Fig.  294).  The  eggs  are  not  dif- 
ferent from  those  formed  within  the  archegonia  of  Bryo- 


Fig.  295.  Antheridium  of  Pteris  (B),  showing  wall  cells  (a),  opening  for  escape  of 
sperm  mother  cells  (e),  escaped  mother  cells  (c),  sperms  free  from  mother  cells 
(b),  showing  spiral  and  multiciliate  character.— Caldwell. 


phytes,  but  the  sperms  are  very  different.  The  Bryophyte 
sperm  has  a  small  body  and  two  long  cilia,  while  the  Pteri- 
dophyte  sperm  has  a  long  spirally  coiled  body,  blunt  behind 
and  tapering  to  a  point  in  front,  where  numerous  cilia  are 
developed  (Fig.  295).  It  is,  therefore,  a  large,  spirally  coiled, 
multiciliate  sperm,  and  is  quite  characteristic  of  all  Pterido- 
phytes  excepting  the  Club-mosses. 

When  the  prothallia  are  developing  the  antheridia  begin 


PTERIDOPHYTES  325 

to  appear  very  early,  and  later  the  archegonia.  If  the  pro- 
thallium  is  poorly  nourished,  only  antheridia  appear;  it 
needs  to  be  well  developed  and  nourished  to  develop  arche- 
gonia. There  seems  to  be  a  very  definite  relation,  there- 
fore, between  nutrition  and  the  development  of  the  two  sex 
organs,  a  fact  which  must  be  remembered  in  connection 
with  certain  later  developments. 

213.  The  sporophyte. — This  complex  body  is  differ- 
entiated into  root,  stem,  and  leaf,  and  is  more  highly 
organized  than  any  plant  body  heretofore  mentioned  (Fig. 
296). 

In  most  of  the  Ferns  the  stem  is  subterranean  and  dor- 
si  ventral  (Fig.  296),  but  in  the  "  tree  ferns  "  of  the  tropics 
it  forms  an  erect,  aerial  shaft  bearing  a  crown  of  leaves 
(Fig.  297).  In  the  other  groups  of  Pteridophytes  there  are 
also  aerial  stems,  both  erect  and  prostrate.  The  stem  is 
complex  in  structure,  the  cells  being  organized  into  differ- 
ent "  tissue  systems,"  prominent  among  which  is  the  vascu- 
lar system. 

One  of  the  peculiarities  of  ordinary  fern  leaves  is  that 
the  vein  system  of  the  leaves  branches  dichotomously,  the 
forking  veins  being  very  conspicuous  (Fig.  298).  Another 
fern  habit  is  that  the  leaves  in  expanding  seem  to  unroll 
from  the  base,  as  though  they  had  been  rolled  from  the 
apex  downward,  the  apex  being  in  the  centre  of  the  roll 
(Fig.  296).  This  habit  is  spoken  of  as  circinate,  from  a 
word  meaning  "  circle  "  or  "  coil,"  and  circinate  leaves  when 
unrolling  have  a  crozier-like  tip.  The  arrangement  of 
leaves  in  bud  is  called  vernation  ("  spring  condition  "),  and 
therefore  the  Ferns  are  said  to  have  circinate  vernation. 
The  combination  of  dichotomous  venation  and  circinate 
vernation  is  very  characteristic  of  Ferns. 

214.  Sporangia. — The  sporangia  are  borne  by  the  leaves, 
generally  upon  the  under  surface,  and  are  usually  closely 
associated  with  the  veins,  and  organized  into  groups  of  defi- 
nite form  known  as  sori.     A  sorus  may  be  round  or  elon- 


Fig.  296.  A  fern  (Aspidinm),  showing  three  large  branching  leaves  coming  from  a 
horizontal  subterranean  stem  (rootstock);  young  leaves  are  also  shown,  which 
show  circinate  vernation.  The  stem,  young  leaves,  and  petioles  of  the  large 
leaves  are  thickly  covered  with  protecting  hairs.  The  stem  gives  rise  to  numerous 
small  roots  from  its  lower  surface.  The  figure  marked  3  represents  the  under  sur- 
face of  a  portion  of  the  leaf,  showing  seven  sori  with  shield-like  indusia;  at  5  is 
represented  a  section  through  a  sorus,  showing  the  sporangia  attached  and  pro- 
tected by  the  indusium;  while  at  6  is  re])resented  a  single  sporangium  opening 
and  discharging  its  spores,  the  heavy  annulus  extending  along  the  back  and  over 
the  top.— After  Wossidlo. 


Fig.  297.  A  group  of  tropical  plants.  To  the  left  of  the  center  is  a  tree  fern,  with  its 
slender  columnar  stem  and  crown  of  large  leaves.  The  large-leaved  plants  to  the 
right  are  bananas  (monocotyledons). 


328 


PLANT   STUDIES 


gated,  and  is  usually  covered  by  a  delicate  flap  (indusium) 
which  arises  from  the  epidermis  (Fig.  296).  Occasionally 
the  sori  are  extended  along  the  under  surface  of  the  mar- 
gin of  the  leaf,  as  in  maidenhair  fern  [Adiantum)^  and  the 
common  brake  (Pteris),  in  which  case  they  are  protected 
by  the  inrolled  margin  (Fig.  298),  which  may  be  called  a 
"false  indusium." 

It  is  evident  that  such  leaves  are  doing  two  distinct 
kinds  of  work — chlorophyll  work  and  spore  formation. 
This  is  true  of  most  of  the  ordinary  Ferns,  but  some  of 

them  show  a  tendency  to  divide 
the  work.  Certain  leaves,  or 
certain  leaf-branches,  produce 
spores  and  do  no  chlorophyll 
work,  while  others  do  chloro- 
phyll work  and  produce  no 
spores.  This  differentiation  in 
the  leaves  or  leaf-regions  is  in- 
dicated by  appropriate  names. 
Those  leaves  which  produce 
only  spores  are  called  sporo- 
phylls,  meaning  "  spore  leaves," 
while  the  leaf  branches  thus 
set  apart  are  called  sporophyll 
branches.  Those  leaves  which 
only  do  chlorophyll  work  are 
called  foliage  leaves  ;  and  such 
branches  are  foliage  branches. 
As  sporophylls  are  not  called 
upon  for  chlorophyll  work  they 
often  become  much  modified,  being  much  more  compact, 
and  not  at  all  resembling  the  foliage  leaves.  Such  a  differ- 
entiation may  be  seen  in  the  ostrich  fern  and  sensitive 
fern  (Onoclea)  (Fig.  299),  the  climbing  fern  (Lygodium), 
the  royal  fern  {Osmu?ida),  the  moonwort  {BotrycMum) 
(Fig.  300),  and  the  adder's  tongue  (Ophioglossum). 


Fig.  298.  Leaflets  of  two  common 
ferns :  A,  the  common  brake 
(Pteris) ;  B,  maidenhair  {Adian- 
turri) ;  both  showing  sori  borne 
at  the  margin  and  protected  by 
the  infolded  margin,  which  thus 
forms  a  false  indusium.— Cald- 
well. 


Fig.  299.     The  genpitive  fern  (Onoclm  sensibilis),  sho^vhv-  differentiation  of  foliage 
leaves  and  eporophylla.— From  "Field,  Forest,  and  Wayside  Flowers." 


330 


PLANT   STUDIES 


"VT 


An  ordinary  fern  sporangium  consists  of  a  slender  stalk 
and  a  bulbous  top  which  is  the  spore  case  (Fig.  296,  6). 
This  case  has  a  delicate  wall  formed  of 
a  single  layer  of  cells,  and  extending 
around  it  from  the  stalk  and  nearly  to 
the  stalk  again,  like  a  meridian  line  about 
a  globe,  is  a  row  of  peculiar  cells  with 
thick  walls,  forming  a  heavy  ring,  called 
the  anmdus.  The  annulus  is  like  a  bent 
spring,  and  when  the  delicate  wall  be- 
comes yielding  the  spring  straightens 
violently,  the  wall  is  torn,  and  in  the  re- 
coil the  spores  are  discharged  with  consid- 
erable force  (Fig.  301).  This  discharge 
of  fern  spores  may  be  seen  by  placing 
some  sporangia  upon  a  moist  slide,  and 
under  a  low  power  watching  them  as  they 
dry  and  burst. 

215.  Heterospory. — This  phenomenon 
appears  first  among  Pteridophytes,  but  it 
is  not  characteristic  of  them,  being  en- 
tirely absent  from  the  true  Ferns,  which 
far  outnumber  all  other  Pteridophytes. 
Its  chief  interest  lies  in  the  fact  that  it 
is  universal  among  the  Spermatophytes, 
and  that  it  represents  the  change  which 
leads  to  the  appearance  of  that  high 
group.  It  is  impossible  to  understand 
the  greatest  group  of  plants,  therefore, 
without  knowing  something  about  heter- 
ospory. As  it  begins  in  simple  fashion 
among  Pteridophytes,  and  is  probably 
the  greatest  contribution  they  have  made 
to  the  evolution  of  the  plant  kingdom, 
unless  it  be  the  leafy  sporophyte,  it  is  best  explained 
here. 


Fig.  300.  A  moonwort 
{Botrychium),  show- 
ing the  leaf  dififerenti- 
ated  into  foliage  and 
sporophyll  branches. 
—After     Strasbur- 


PTERIDOPIIYTES 


331 


In  the  ordinary  Ferns  all  the  spores  in  the  sporangia 
are  alike,  and  when  they  germinate  each  spore  produces  a 
prothallium  upon  which  both  antheridia  and  archegonia 
appear. 

In  some  Pteridophytes,  however,  there  is  a  decided  dif- 
ference in  the  size  of  the  spores,  some  being  quite  small  and 


Fig.  301.  A  series  showing  the  dehiscence  of  a  fern  sporangium,  the  rupture  of  the 
wall,  the  straightening  and  bending  back  of  the  annulus,  and  the  recoil.— After 
Atkinson. 


others  relatively  large,  the  small  ones  producing  male  game- 
tophytes  (prothallia  with  antheridia),  and  the  large  ones 
female  gametophytes  (prothallia  with  archegonia).  When 
asexual  spores  differ  thus  permanently  in  size,  and  give  rise 


332  PLANT   STUDIES 

to  gametophytes  of  different  sexes,  we  have  the  condition 
called  heterospory  ("  spores  different "),  and  such  plants  are 
called  heterosporoiis  (Fig.  307).  In  contrast  with  hetero- 
sporous  plants,  those  in  wliich  the  asexual  spores  appear 
alike  are  called  homosporous^  or  sometimes  isosporous^  both 
terms  meaning  "  spores  similar."  The  corresponding  noun 
form  is  liomospory  or  isospory.  Bryophytes  and  most  Pteri- 
dophytes  are  homosporous,  while  some  Pteridophytes  and 
all  Spermatophytes  are  heterosporous. 

It  is  convenient  to  distinguish  by  suitable  names  the 
two  kinds  of  asexual  spores  produced  by  the  sporangia  of 
heterosporous  plants  (Fig.  307).  The  large  ones  are  called 
megaspores^  or  by  some  writers  macrospores^  both  terms 
meaning  "  large  spores  "  ;  the  small  ones  are  called  7nicro- 
spores,  or  "  small  spores."  It  should  be  remembered  that 
megaspores  always  produce  female  gametophytes,  and  mi- 
crospores male  gametophytes. 

This  differentiation  does  not  end  with  the  spores,  but 
soon  involves  the  sporangia  (Fig.  307).  Some  sporangia 
produce  only  megaspores,  and  are  called  megasporangia ; 
others  produce  only  microspores,  and  are  called  microspo- 
rangia.  It  is  important  to  note  that  while  microsporangia 
usually  produce  numerous  microspores,  the  megasporangia 
produce  much  fewer  megaspores,  the  tendency  being  to 
diminish  the  number  and  increase  the  size,  until  finally 
there  are  megasporangia  which  produce  but  a  single  large 
megaspore. 

A  formula  may  indicate  the  life  history  of  a  hetero- 
sporous plant.  The  formula  of  homosporous  plants  with 
alternation  of  generations  (Bryophytes  and  most  Pterido- 
phytes) was  given  as  follows  (§  197) : 

Gizg> 0— S— 0— G=:8> o— S— 0— Gzzg> o— S,  etc. 

In  the  case  of  heterosporous  plants  (some  Pterido- 
phytes and  all  Spermatophytes)  it  would  be  modified  as 
follows : 

g=8>o— Szzg=g:=g>o— S=8izg-.g>o— S,  etc. 


PTERIDOPIIYTES  333 

In  this  case  two  gametophytes  are  involved,  one  pro- 
ducing a  sperm,  the  other  an  egg,  which  fuse  and  form  the 
oospore,  which  in  germination  produces  the  sporophyte, 
which  produces  two  kinds  of  asexual  spores  (megaspores 
and  microspores),  which  in  germination  produce  the  two 
gametophytes  again. 

One  additional  fact  connected  with  heterospory  should 
be  mentioned,  and  that  is  the  great  reduction  of  the  gam- 
etophyte.  In  the  homosporous  ferns  the  spore  develops 
a  small  but  free  and  independent  prothallium  which  pro- 
duces both  sex  organs.  When  in  heterosporous  plants  this 
work  of  producing  sex  organs  is  divided  between  two  gam- 
etophytes they  become  very  much  reduced  in  size  and  lose 
their  freedom  and  independence.  They  are  so  small  that 
they  do  not  escape  entirely,  if  at  all,  from  the  embrace  of 
the  spores  which  produce  them,  and  are  mainly  dependent 
for  their  nourishment  upon  the  food  stored  up  in  the  spores. 


CHAPTER   XXII 

THE  GREAT  GROUPS  OF  PTERIDOPHYTES 

216.  The  great  groups. — At  least  three  independent  lines 
of  Pteridophytes  are  recognized  :  (1)  Filicales  (Ferns), 
(2)  Equisetales  (Scouring  rushes.  Horsetails),  and  (3)  Ly- 
copodiales  (Cluh-mosses).  The  Ferns  are  much  the  most 
abundant,  the  Club-mosses  are  represented  by  a  few  hun- 
dred forms,  while  the  Horsetails  include  only  about  twenty- 
five  species.  These  three  great  groups  are  so  unlike  that 
they  hardly  seem  to  belong  together  in  the  same  division 
of  the  plant  kingdom. 

Filicales  {Ferns) 

217.  General  characters. — The  Ferns  were  used  in  the 
preceding  chapter  as  types  of  Pteridophytes,  so  that  little 
need  be  added.  They  well  deserve  to  stand  as  types,  as 
they  contain  about  four  thousand  of  the  four  thousand  five 
hundred  species  belonging  to  Pteridophytes.  Although 
found  in  considerable  numbers  in  temperate  regions,  their 
chief  display  is  in  the  tropics,  where  they  form  a  striking 
and  characteristic  feature  of  the  vegetation.  In  the  trop- 
ics not  only  are  great  masses  of  the  low  forms  to  be  seen, 
from  those  with  delicate  and  filmy  moss  like  leaves  to  those 
with  huge  leaves,  but  also  tree  forms  with  cylindrical 
trunks  encased  by  the  rough  remnants  of  fallen  leaves  and 
sometimes  rising  to  a  height  of  thirty-five  to  forty-five 
feet,  with  a  great  crown  of  leaves  fifteen  to  twenty  feet 
long  (Fig.  297). 

384 


336 


PLANT   STUDIES 


There  are  also  epiphytic  forms  (air  plants) — that  is, 
those  which  perch  "  upon  other  plants "  but  derive  no 
nourishment  from  them  (Fig.  95).  This  habit  belongs 
chiefly  to  the  warm  and  moist  tropics,  where  the  plants 
can  absorb  sufficient  moisture  from  the  air  without  send- 
ing roots  into  the  soil.  In  this  way  many  of  the  tropical 
ferns  are  found  growing  upon  living  and  dead  trees  and 
other  plants.  In  the  temperate  regions  the  chief  epi- 
phytes are  Lichens,  Liverworts,  and  Mosses,  the  Ferns  be- 
ing chiefly  found  in  moist  woods  and  ravines  (Fig.  302), 
although  a  number  grow  in  comparatively  dry  and  exposed 
situations,  sometimes  covering  extensive  areas,  as  the  com- 
mon brake  {Pteris). 

The  Filicales  differ  from  the  other  groups  of  Pterido- 
phytes  chiefly  in  having  few  large  leaves,  which  do  chloro- 
phyll work  and  bear  sporangia.  In  a  few  of  them  there  is  a 
differentiation  of  functions  in  foliage  branches  and  sporo- 
phyll  branches  (Figs.  299,  300),  but  even  this  is  excep- 
tional. Another  distinction  is  that  the  stems  are  un- 
branched. 

218.  Origin  of  sporangia. — An  important  feature  in  the 
Ferns  is  the  origin  of  the  sporangia.  In  some  of  them  a 
sporangium  is  developed  from  a  single  epidermal  cell  of 
the  leaf,  and  is  an  entirely  superficial  and  generally  stalked 
affair  (Fig.  296,  S) ;  in  others  the  sporangium  in  its  devel- 
opment involves  several  epidermal  and  deeper  cells  of  the 
leaf,  and  is  more  or  less  of  an  imbedded  affair.  In  the  first 
case  the  ferns  are  said  to  be  leptosporangiate ;  in  the  sec- 
ond case  they  are  eusporangiate. 

Another  small  but  interesting  group  of  Ferns  includes 
the  "  Water-ferns,"  fioating  forms  or  sometimes  on  muddy 
flats.  The  common  Marsilia  may  be  taken  as  a  type  (Fig. 
303).  The  slender  creeping  stem  sends  down  numerous 
roots  into  the  mucky  soil,  and  at  intervals  gives  rise  to  a 
comparatively  large  leaf.  This  leaf  has  a  long  erect  petiole 
and  a  blade  of  four  spreading  wedge-shaped  leaflets  like  a 


THE   GREAT   GROUPS   OF   PTERIDOPIIYTES 


337 


"  four-leaved  clover."  The  dichotomous  venation  and  cir- 
cinate  vernation  at  once  suggest  the  fern  alliance.  From 
near  the  base  of  the  petiole  another 
leaf  branch  arises,  in  which  the  blade 
is  modified  as  a  sporophyll.  In  this 
case  the  sporophyll  incloses  the  spo- 
rangia and  becomes  hard  and  nut- 
like.    Another  common  form  is  the 


Fig.  303.  A  water-fern  (Marsilia), 
showing  horizontal  stem,  with 
descending  roots,  and  ascend- 
ing leaves  ;  a,  a  young  leaf 
showing  circinate  vernation ; 
«,«,8porophyll  branches  ("spo- 
rocarps  ").— After  Bisouofp. 


Fig.  304.  One  of  the  floating  water-ferns  (Sal- 
vi7iia),  showing  side  view  (.-1)  and  view  from 
above  (5).  The  dangling  root-like  processes 
are  the  modified  submerged  leaves.  In  A, 
near  the  top  of  the  cluster  of  submerged 
leaves,  some  sporophyll  branches  ("si)oro- 
carps  ")  may  be  seen.— After  Bischofp. 


floating  Salvinia  (Fig.  304).  The  chief  interest  lies  in  the 
fact  that  the  water-ferns  are  heterosporous.  As  they  are 
leptosporangiate  they  are  thought  to  have  been  derived  from 
the  ordinary  leptosporangiate  Ferns,  which  are  homosporous. 


Equisetales  {Horsetails  or  Scouring  rnslies) 

210.  General  characters. — The  twenty-five  forms  now  rep- 
resenting this  great  group  belong  to  a  single  genus  {Equise- 


338  PLANT   STUDIES 

tum^  meaning  "  horsetail "),  but  they  are  but  the  linger- 
ing remnants  of  an  abundant  flora  which  lived  in  the  time 
of  the  Coal-measures,  and  helped  to  form  the  forest  vegeta- 
tion. The  living  forms  are  small  and  inconspicuous,  but 
very  characteristic  in  appearance.  They  grow  in  moist  or 
dry  places,  sometimes  in  great  abundance  (Fig.  305). 

The  stem  is  slender  and  conspicuously  jointed,  the  joints 
separating  easily ;  it  is  also  green,  and  fluted  with  small 
longitudinal  ridges ;  and  there  is  such  an  abundant  deposit 
of  silica  in  the  epidermis  that  the  plants  feel  rough.  This 
last  property  suggested  its  former  use  in  scouring,  and  its 
name  "  scouring  rush."  At  each  joint  is  a  sheath  of  minute 
leaves,  more  or  less  coalesced,  the  individual  leaves  some- 
times being  indicated  only  by  minute  teeth.  This  arrange- 
ment of  leaves  in  a  circle  about  the  joint  is  called  the  cyclic 
arrangement,  or  sometimes  the  wJiorled  arrangement,  each 
such  set  of  leaves  being  called  a  cycle  or  a  icliorl.  These 
leaves  contain  no  chlorophyll  and  have  evidently  abandoned 
chlorophyll  work,  which  is  carried  on  by  the  green  stem. 
Such  leaves  are  known  as  scales^  to  distinguish  them  from 
foliage  leaves.  The  aerial  stem  (really  a  branch)  is  either 
simple  or  profusely  branched  (Fig.  305).  In  the  species 
illustrated  the  early  aerial  branches  are  simple,  usually  not 
green,  and  bear  the  strobili ;  while  the  later  branches  are 
sterile,  profusely  branched,  and  green. 

220.  The  strobilus. — One  of  the  distinguishing  charac- 
ters of  the  group  is  that  chlorophyll-work  and  spore-forma- 
tion are  completely  differentiated.  Although  the  foliage 
leaves  are  reduced  to  scales,  and  the  chlorophyll-work  is 
done  by  the  stem,  there  are  well-organized  sporophylls. 
The  sporophylls  are  grouped  close  together  at  the  end  of 
the  stem  in  a  compact  conical  cluster  which  is  called  a 
strobilus,  the  Latin  name  for  "  pine  cone,"  which  this  clus- 
ter of  sporophylls  resembles  (Fig.  305). 

Each  sporophyll  consists  of  a  stalk-like  portion  and  a 
shield-like    (peltate)   top.     Beneath  the   shield  hang  the 


Fig.  305.  Eqinsetum  arvense,  a  common  horsetail:  i,  three  fertile  shoots  rising  from 
the  dorsiventral  stem,  showing  the  cycles  of  coalesced  scale-leaves  at  the  joints 
and  the  terminal  strobili  with  numerous  sporophylls,  that  at  a  being  mature;  5, 
a  sterile  shoot  from  the  same  stem,  showing  branching;  5,  a  single  peltate  sporo- 
phyll  bearing  sporangia;  U,  view  of  sporophyll  from  beneath,  showing  dehiscence 
of  sporangia;  5,  6",  7,  spores,  showing  the  unwinding  of  the  outer  coat,  which  aide 
in  dispersal.— After  Wossidlo. 


340  PLANT   STUDIES 

sporangia,  which  produce  spores  of  but  one  kind,  hence 
these  plants  are  homosporous ;  and  as  the  sporangia  origi- 
nate in  eusporangiate  fashion,  Equisetum  has  the  homospo- 
rous-eusporangiate  combination  shown  by  one  of  the  Fern 
groups.  It  is  interesting  to  know,  however,  that  some  of 
the  ancient,  more  highly  organized  members  of  this  group 
were  heterosporous,  and  that  the  present  forms  have  dioe- 
cious gametophytes. 

Lycopodiales  (Club-mosses) 

221.  General  characters. — This  group  is  now  represented 
by  about  five  hundred  species,  most  of  which  belong  to 
the  two  genera  Lycopodium  and  Selagmella^  the  latter 
being  much  the  larger  genus.  The  plants  have  slender, 
branching,  prostrate,  or  erect  stems  completely  clothed 
with  small  foliage  leaves,  having  a  general  moss-like  ap- 
pearance (Figs.  306,  307).  Often  the  erect  branches  are 
terminated  by  conspicuous  conical  or  cylindrical  strobili, 
which  are  the  "  clubs "  that  enter  into  the  name  "  Club- 
mosses."  There  is  also  a  certain  kind  of  resemblance  to 
miniature  pines,  so  that  the  name  "  Ground-pines  "  is  some- 
times used. 

Lycopodiales  were  once  much  more  abundant  than  now, 
and  more  highly  organized,  forming  a  conspicuous  part  of 
the  forest  vegetation  of  the  Coal-measures. 

One  of  the  distinguishing  marks  of  the  group  is  that  the 
sperm  does  not  resemble  that  of  the  other  Pteridophytes, 
but  is  of  the  Bryophyte  type  (Fig.  277)  ;  that  is,  it  con- 
sists of  a  small  body  with  two  cilia,  instead  of  a  large 
spirally  coiled  body  with  many  cilia.  Another  distinguish- 
ing character  is  that  there  is  but  a  single  sporangium  pro- 
duced by  each  sporophyil  (Fig.  306).  This  is  in  marked 
contrast  with  the  Filicales,  whose  leaves  bear  very  numer- 
ous sporangia,  and  with  the  Equisetales,  whose  sporophylls 
bear  several  sporangia. 


Fig.  306.  A  common  club-moss  (Lycopodinm  davatmn):  L  tho 
horizontal  stem  giving  rise  to  roots  and  to  erect  branches 
smgle  sporophyll  with  its  sporangium;  3,  spores,  much  magnified%After  Vos 

BIDLO. 


whole  plant,  showing 
bearing  strobili:  ;>  >i 


Fig.  307.  Selaginella  Martensii :  A,  branch  bearing  strobili;  B,  a  microsporophyll 
with  a  microsporangium,  showing  microspores  through  a  rupture  in  the  wall;  C, 
a  megasporophyll  with  a  megasporangium  ;  D,  megaspores  ;  E,  microspores. — 

GOLDBEKGER. 


CHAPTEE  XXIII 

SPERMATOPHYTES :    GYMNOSPERMS 

222.  Summary  from  Pteridophytes. — In  considering  the 
important  contributions  of  Pteridophytes  to  the  evolution 
of  the  plant  kingdom  the  following  seem  worthy  of  note  : 

(1)  Prominence  of  sporophyte  mid  development  of  vascu- 
lar system. — This  prominence  is  associated  with  the  display 
of  leaves  for  chlorophyll  work,  and  the  leaves  necessitate 
the  work  of  conduction,  v/hich  is  arranged  for  by  the  vas- 
cular system.     This  fact  is  true  of  the  whole  group. 

(2)  Differentiation  of  sporopliylls. — The  appearance  of 
sporophylls  as  distinct  from  foliage  leaves,  and  their  or- 
ganization into  the  cluster  known  as  the  strobilus,  are  facts 
of  prime  importance.  This  differentiation  appears  more  or 
less  in  all  the  great  groups,  but  the  strobilus  is  distinct  only 
in  Horsetails  and  Club-mosses. 

(3)  Introduction  of  lieterospory  and  reduction  of  gameto- 
phytes. — Heterospory  appears  independently  in  all  of  the 
three  great  groups — in  the  water-ferns  among  the  Fili- 
cales,  in  the  ancient  horsetails  among  the  Equisetales,  and 
in  Selaginella  and  Isoetes  among  Lycopodiales.  All  the 
other  Pteridophytes,  and  therefore  the  great  majority  of 
them,  are  homosporous.  The  importance  of  the  appear- 
ance of  heterospory  lies  in  the  fact  that  it  leads  to  the 
development  of  Spermatophytes,  and  associated  with  it  is 
a  great  reduction  of  the  gametophytes,  which  project  little, 
if  at  all,  from  the  spores  whicli  produce  them. 

223.  Summary  of  the  four  groups. — It  may  be  well  in  this 
connection  to  give  certain  prominent  characters  which  will 

23  343 


344 


PLANT   STUDIES 


serve  to  distinguish  the  four  great  groups  of  plants.  It 
must  not  be  supposed  that  these  are  the  only  characters, 
or  even  the  most  important  ones  in  every  case,  but  they 
are  convenient  for  our  purpose.  Two  characters  are  given 
for  each  of  the  first  three  groups — one  a  positive  character 
which  belongs  to  it,  the  other  a  negative  character  which 
distinguishes  it  from  the  group  above,  and  becomes  the 
positive  character  of  that  group. 

(1)  Tliallopliytes, — Thallus  body,  but  no  archegonia. 

(2)  Bryo2)liytes.—A.TchQgoTn^,  but  no  vascular  system. 

(3)  Ftericlophytes.— Vascular  system,  but  no  seeds. 

(4)  Sj^ermatophytes. — Seeds. 

224.  General  characters  of  Spermatophytes. — This  is  the 
greatest  group  of  plants  in  rank  and  in  display.  So  con- 
spicuous are  they,  and  so  much  do  they  enter  into  our 
experience,  that  they  have  often  been  studied  as  "botany," 
to  the  exclusion  of  the  other  groups.  The  lower  groups 
are  not  meiely  necessary  to  fill  out  any  general  view  of  the 
plant  kingdom,  but  they  are  absolutely  essential  to  an 
understanding  of  the  structures  of  the  highest  group. 

This  great  dominant  group  has  received  a  variety  of 
names.  Sometimes  they  are  called  Antliopliytes,  meaning 
''Flowering  plants,"  with  the  idea  that  they  are  distin- 
guished by  the  production  of  ''  flowers."  A  flower  is  diffi- 
cult to  define,  but  in  the  popular  sense  all  Spermatophytes 
do  not  produce  flowers,  while  in  another  sense  the  strobilus 
of  Pteridophytes  is  a  flower.  Hence  the  flower  does  not 
accurately  limit  the  group,  and  the  name  Anthophytes  is 
not  in  general  use.  Much  more  commonly  the  group  is 
called  Phanerogams  (sometimes  corrupted  into  Phaenogams 
or  even  Phenogams),  meaning  '^  evident  sexual  reproduc- 
tion." At  the  time  this  name  was  proposed  all  the  other 
groups  were  called  Cryptogams,  meaning  "hidden  sexual 
reproduction."  It  is  a  curious  fact  that  the  names  ought 
to  have  been  reversed,  for  sexual  reproduction  is  much  more 
evident  in  Cryptogams  than  in  Phanerogams,  the  mistake 


SPEKMATOPHYTES :  GYMNOSPERMS         345 

arising  from  the  fact  that  what  were  supposed  to  be  sexual 
organs  in  Phanerogams  have  proved  not  to  be  such.  The 
name  Phanerogam,  therefore,  is  being  generally  abandoned  ; 
but  the  name  Cryptogam  is  a  useful  one  when  the  lower 
groups  are  to  be  referred  to ;  and  the  Pteridophytes  are 
still  very  frequently  called  the  Vascular  Cryptogams,  The 
most  distinguishing  mark  of  the  group  seems  to  be  the 
production  of  seeds,  and  hence  the  name  Spermatopliytes, 
or  ^^  Seed-plants,"  is  coming  into  general  use. 

The  seed  can  be  better  defined  after  its  development 
has  been  described,  but  it  results  from  the  fact  that  in  this 
group  the  single  megaspore  is  never  discharged  from  its 
megasporangium,  but  germinates  just  where  it  is  devel- 
oped. The  great  fact  connected  with  the  group,  therefore, 
is  the  retention  of  the  megaspore,  which  results  in  a  seed. 
The  full  meaning  of  this  will  appear  later. 

There  are  two  very  independent  lines  of  Seed-plants, 
the  Gymnosperms  and  the  Angiosperms.  The  first  name 
means  '^  naked  seeds,"  referring  to  the  fact  that  the  seeds 
are  always  exposed;  the  second  means  ** inclosed  seeds," 
as  the  seeds  are  inclosed  in  a  seed  vessel. 

Gymnosperms 

225.  General  characters. — The  most  familiar  Gymnosperms 
in  temperate  regions  are  the  pines,  spruces,  hemlocks, 
cedars,  etc.,  the  group  so  commonly  called  ^'evergreens." 
It  is  an  ancient  tree  group,  for  its  representatives  were 
associated  with  the  giant  club-mosses  and  horsetails  in 
the  forest  vegetation  of  the  Coal-measures.  Only-  about 
four  hundred  species  exist  to-day  as  a  remnant  of  its  for- 
mer display,  although  the  pines  still  form  extensive  forests. 
The  group  is  so  diversified  in  its  structure  that  all  forms 
can  not  be  included  in  a  single  description.  The  common 
pine  {Piiius),  therefore,  will  be  taken  as  a  type,  to  show 
the  general  Gymnosperm  character. 


346  PLANT   STUDIES 

226.  The  plant  body. — The  great  body  of  the  plant, 
often  forming  a  large  tree,  is  the  sporophyte ;  in  fact,  the 
gametophytes  are  not  visible  to  ordinary  observation.  It 
should  be  remembered  that  the  sporophyte  is  distinctly  a 
sexless  generation,  and  that  it  develops  no  sex  organs. 
This  great  sporophyte  body  is  elaborately  organized  for 
nutritive  work,  with  its  roots,  stems,  and  leaves.  These 
organs  are  very  complex  in  structure,  being  made  up  of 
various  tissue  systems  that  are  organized  for  special  kinds 
of  work.  The  leaves  are  the  most  variable  organs,  being 
differentiated  into  three  distinct  kinds :  (1)  foliage  leaves, 
(2)  scales,  and  (3)  sporophylls. 

227.  Sporophylls. — The  sporophylls  are  leaves  set  apart 
to  produce  sporangia,  and  in  the  pine  they  are  arranged 
in  a  strobilus,  as  in  the  Horsetails  and  Club-mosses.  As 
the  group  is  heterosporous,  however,  there  are  two  kinds 
of  sporophylls  and  two  kinds  of  strobili.  One  kind  of 
strobilus  is  made  up  of  megasporophylls  bearing  mega- 
sporangia  ;  the  other  is  made  up  of  microsporophylls  bear- 
ing microsporangia.  These  strobili  are  often  spoken  of  as 
the  "  flowers "  of  the  pine,  but  if  these  are  flowers,  so  are 
the  strobili  of  Horsetails  and  Club-mosses. 

228.  Microsporophylls. — In  the  pines  the  strobilus  com- 
posed of  microsporophylls  is  comparatively  small  (Figs. 
308,  d,  309).  Each  sporophyll  is  like  a  scale  leaf,  is  nar- 
rowed at  the  base,  and  upon  the  lower  surface  are  borne 
two  prominent  sporangia,  which  of  course  are  microspo- 
rangia, and  contain  microspores  (Fig.  309). 

These  structures  of  Seed-plants  all  received  names 
before  they  were  identified  with  the  corresponding  struc- 
tures of  the  lower  groups.  The  microsporophyll  was  called  a 
stamen^  the  microsporangia  pollen-sacs^  and  the  microspores 
pollen-grains^  or  simply  pollen.  These  names  are  still  very 
convenient  to  use  in  connection  with  the  Spermatophytes, 
but  it  should  be  remembered  that  they  are  simply  other 
names  for  structures  found  in  the  lower  groups. 


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Fig.  308.  Pinus  Laricio,  showing  tip  of  branch  bearing  needle-leaves,  scale-leaves, 
and  cones  (strobili):  a,  very  young  carpellate  cones,  at  time  of  pollination,  borne 
at  tip  of  the  young  shoot  upon  which  new  leaves  are  appearing;  6,  carpellate  cones 
one  year  old;  c,  carpellate  cones  two  years  old,  the  scales  spreading  and  shedding 
the  seeds;  d,  young  shoot  bearing  a  cluster  of  staminate  cones.— Caldwell. 


348 


PLANT   STUDIES 


The  strobilus  composed  of  microsporophylls  may  be 
called  the  staminate  stroMlus — that  is,  one  composed  of 
stamens ;  it  is  often  called  the  staminate  cone,  "  cone " 
being  the  English  translation  of  the  word  "strobilus." 
Frequently  the  staminate  cone  is  spoken  of  as  the  "  male 
cone/'  as  it  was  once  supposed  that  the  stamen  is  the 


Fig.  309.  Staminate  cone  (strobilus)  of  pine  (Finns) :  A,  section  of  cone,  showing 
microsporophylls  (stamens)  bearing  microsporangia;  B,  longitudinal  section  of  a 
single  stamen,  showing  the  large  sporangium  beneath  ;  C,  cross-section  of  a  sta- 
men, showing  the  two  sporangia;  J),  a  single  microspore  (pollen  grain)  much  en- 
larged, showing  the  two  wings,  and  a  male  gametophyte  of  two  cells,  the  lower 
and  larger  (wall  cell)  developing  the  pollen  tube,  the  upper  and  smaller  (genera- 
tive cell)  giving  rise  to  the  sperms. — After  Strasburger. 

male  organ.  This  name  should,  of  course,  be  abandoned, 
as  the  stamen  is  now  known  to  be  a  microsporophyll,  which 
is  an  organ  produced  by  the  sporophyte,  which  never  pro- 
duces sex  organs.  It  should  be  borne  distinctly  in  mind 
that  the  stamen  is  not  a  sex  organ,  for  the  literature  of 
botany  is  full  of  this  old  assumption,  and  the  beginner  is  in 


SPERMATOPIIYTES  :    GYMNOSPERMS 


349 


danger  of  becoming  confused  and  of  forgetting  that  pollen 
grains  are  asexual  spores. 

229.  Megasporophylls. — The  strobili  composed  of  mega- 
sporopliylls  become  much  larger  than  the  others,  forming 


Fig.  310.  Pinus  sylvestris,  showing  mature  cone  partly  sectioned,  and  showinsr  car- 
pels (sq,  sq^,  sq^)  with  seeds  in  their  axils  (g),  in  which  the  embryos  (em)  may  be 
distinguished  ;  A,  a  young  carpel  with  two  megasporangia  ;  B,  an  old  carpel  with 
mature  seeds  (ch),  the  micropyle  being  below  (J/).— After  Bessey. 


the  well-known  cones  so  characteristic  of  pines  and  their 
allies  (Fig.  308,  «,  Z>,  c).  Each  sporophyll  is  somewhat 
leaf-like,  and  at  its  base  upon  the  upper  side  are  two 
megasporangia  (Fig.  310).  It  is  these  sporangia  which  are 
peculiar  in  each  producing  and  retaining  a  solitary  large 
megaspore.     This  megaspore  resembles  a  sac-like  cavity  in 


350 


PLANT   STUDIES 


the  body  of  the  sporangium  (Fig.  311,  ^),  and  was  at  first 
not  recognized  as  being  a  spore. 

These  structures  had  also  received  names  before  they 
were  identified  with  the  corresponding  structures  of  the 
lower  groups.  The  megasporophyll  was  called  a  carpel, 
the  megasporangia  ovules,  and  the  megaspore  an  embryo- 
sac,  because  the  young  embryo  was  observed  to  develop 
within  it  (Fig.  310,  em). 

The  strobilus  of  megasporophylls,  therefore,  may  be 
called  the  carpellate  strobilus  or  carpellate  cone.  As  the 
carpel  enters  into  the  organization  of  a  structure  known  as 
the  pistil,  to  be  described  later,  the  cone  is  often  called 
the  pistillate  cone.  As  the  staminate  cone  is  sometimes 
wrongly  called  a  "male  cone,"  so  the  carpellate  cone  is 
wrongly  called  a  "female  cone,"  the 
old  idea  being  that  the  carpel  with 
its  ovules  represented  the  female  sex 
organ. 

The  structure  of  the  megaspo- 
rangium,  or  ovule,  must  be  known. 
The  main  body  is  the  nucellus  (Figs. 
311,  c,  312,  nc) ;  this  sends  out  from 
near  its  base  an  outer  membrane 
(integument)  which  is  distinct  above 
(Figs.  311,  b,  312,  i),  covering  the  main 
part  of  the  nucellus  and  projecting 
beyond  its  apex  as  a  prominent  neck. 

Fig.  311.     Diagram  of  the  -^  -^  i        i  •    i    +     xi, 

carpel  structures  of  pine,     the  passagc  through  which  to  the  apex 
showing  the  heavy  scale     of  the  uucellus  is  callcd  the  7iiicropyle 

{A)    which     bears     the       //■<•  t  ,  ii  ,     ,,\     /-rt-         o-. -.         \         m 

ovule  (5), in  which  are     ("little   gate")   (Fig.  311,  «).      Ccn- 
seen  the  micropyie  (a),     trally  placed  within  the  body  of  the 

integument  (ft),  nucellus  n  •        j.i  •  -i. 

(c),  embryo-sac  or  mega-       nUCClluS     IS     the      COUSpiCUOUS     CaVlty 

spore  ((Z).— Moore.  called  the  cmbryo-sac  (Fig.   311,  d), 

in    reality    the    retained    megaspore. 

The   relations   between    integument,   micropyie,   nucellus, 

and  embryo-sac  should  be  kept  clearly  in  mind.     In  the 


SPERMATOPHYTES  :    GYMNOSPERMS 


351 


nc 


pine  the  micropyle  is  directed  downward,  toward  the  base 
of  the  sporophyll. 

230.  The  gametophytes. — The  male  and  female  gameto- 
phytes  are  so  small  that  they  develop  entirely  within  the 
spores  (pollen-grain  and 
embryo-sac),  and  there- 
fore can  only  be  observed 
by  the  microscope. 

The  female  gameto- 
phyte  (often  called  "  en- 
dosperm '')  fills  up  the 
large  embryo-sac,  and  on 
its  surface  toward  the 
micropyle  develops  regu- 
lar flask-shaped  arche- 
gonia  (Fig.  312). 

The  male  gameto- 
phyte  is  still  more  re- 
duced, and  is  represented 
by  a  very  few  small  cells 
which  appear  within  the 
pollen  -  grain,  two  of 
which  are  sperm  -  cells. 
These  sperm-cells  must 
reach  the  archegonia, 
and  accordingly  the  pol- 
len-grain sends  out  a  tube 
(poUe7i-tube)^  into  which 
the  sperm-cells  enter,  and 
are  thus  brought  to  the 
archegonia  (Fig.  110). 

231.  Fertilization.  — 
Before  fertilization  can 
take  place  the  pollen-grains  (microspores)  must  be  brought 
as  near  as  possible  to  the  female  gametophyte  with  its  arche- 
gonia.    The  spores  are  formed  in  very  great  abundance, 


Fig.  312.  Diagrammatic  section  through  ovule 
(megasi)orangium)  of  spruce  {Picea),  showing 
integument  (i),  nucellus  {nc),  endosperm  or 
female  gametophyte  (,e)  which  fills  the  large 
megaspore  imbedded  in  the  nucellus,  two 
archegonia  (a)  with  short  neck  (c)  and  venter 
containing  the  egg  (o),  and  position  of  ger- 
minating pollen-grains  or  microspores  (/>> 
whose  tubes  (/)  {)enetraie  the  nucellus  tissue 
and  reach  the  archegonia.— After  Scuimper. 


352 


PLANT   STUDIES 


are  dry  and  powdery,  and  are  scattered  far  and  wide  by  the 
wind.  In  the  pines  and  their  allies  the  pollen-grains  are 
winged  (Fig.  309,  i)),  so  that  they  are  well  organized  for 
wind  distribution.  This  transfer  of  pollen  is  called  pol- 
Unationj  and  those  plants  that  nse  the  wind  as  an  agent  of 
transfer  are  said  to  be  miemopJiilous^  or  "  wind-loving." 

The  pollen  must  reach  the  ovule,  and  to  insure  this  it 
must  fall  like  rain.     To  aid  in  catching  the  falling  pollen 

the  scale-like  carpels  of  the  cone 
spread  apart,  the  pollen -grains 
slide  down  their  sloping  surfaces 
and  collect  in  a  little  drift  at  the 
bottom  of  each  carpel,  where  the 
ovules  are  found  (Fig.  310,  J,  ^). 
The  flaring  lips  of  the  micropyle 
roll  inward  and  outward  as  they 
are  dry  or  moist,  and  by  this  mo- 
tion some  of  the  pollen-grains  are 
caught  and  pressed  down  upon  the 
apex  of  the  nucellus. 

In  this  position  the  pollen-tube 
develops,  crowds  its  way  among 
the  cells  of  the  nucellus,  reaches 
the  wall  of  the  embryo-sac,  and 
penetrating  that,  reaches  the  necks 
of  the  archegonia. 

232.  The  embryo.— By  the  act  of 
fertilization,  an  oospore  is  formed 
within  the  archegonium.  As  it  is  on  the  surface  of  its  food 
supply  (the  endosperm),  it  first  develops  a  long  cylindrical 
process  (suspensor),  which  penetrates  the  endosperm  and 
develops  the  embryo  at  its  tip.  In  this  way  the  embryo  lies 
imbedded  in  the  midst  of  its  food  supply  (Fig.  313). 

233.  The  seed. — While  the  embryo  is  developing,  some 
important  changes  are  taking  place  in  the  ovule  outside  of 
the  endosperm.     The  most  noteworthy  is  the  change  which 


Fig.  313.  Embryos  of  pine :  A, 
very  young  embryos  (ka)  at  the 
tips  of  long  and  contorted  sus- 
pensors  (s) ;  B,  older  embryo, 
showing  attachment  to  suspen- 
sor  (s),  the  extensive  root  sheath 
(wk),  root  tip  (ws),  stem  tip 
(v),  and  cotyledons  (c).— After 
Strasburger. 


SPEKMATOPHYTES  :    GYMNOSPERMS 


353 


transforms    the   integument   into   a   hard   bony   covering, 

known  as  the  seed  coat^  or  testa  (Fig. 

314).      The   development  of  this   testa 

hermetically  seals  the  structures  within, 

further   development   and  activity   are 

checked,  and  the  living  cells  pass  into 

the  resting  condition.     This  protected  structure  with  its 

dormant  cells  is  the  seed. 

The  organization  of  the  seed  checks  the  growth  of  the 
embryo,  and  this  development  within  the  seed  is  known  as 


Fig,  314.     Pine  seed. 


^^^^ 


Fig.  315.    Pine  seedlings,  showing  the  long  hypocotyl  and  the  nnmerous  cotyledons, 
with  the  old  seed  case  still  attached.— After  Atkinson. 


35i  PLA^^T   STUDIES 

the  intra-seminal  develojoment.  In  this  condition  the  em- 
bryo may  continue  for  a  very  long  time,  and  it  is  a  ques- 
tion whether  it  is  death  or  suspended  animation.  Is  a 
seed  alive  ?  is  not  an  easy  question  to  answer,  for  it  may 
be  kept  in  a  dried-out  condition  for  years,  and  then  when 
placed  in  suitable  conditions  awaken  and  put  forth  a  liv- 
ing plant. 

This  "  awakening  "  of  the  seed  is  spoken  of  as  its  "  ger- 
mination," but  this  must  not  be  confused  with  the  germi- 
nation of  a  spore,  which  is  real  germination.  In  the  case 
of  the  seed  an  oospore  has  germinated  and  formed  an  em- 
bryo, which  stops  growing  for  a  time,  and  then  resumes  it. 
This  resumption  of  growth  is  not  germination,  but  is  what 
happens  when  a  seed  is  said  to  "  germinate."  This  second 
period  of  development  is  known  as  the  extra-seminal^  for  it 
is  inaugurated  by  the  escape  of  the  sporophyte  from  the 
seed  coats  (Fig.  315). 

234.  The  great  groups  of  Gymnosperms. — There  are  at 
least  four  living  groups  of  Gymnosperms,  and  two  or  three 
extinct  ones.  The  groups  differ  so  widely  from  one  an- 
other in  habit  as  to  show  that  Gymnosperms  can  be  very 
much  diversified.  They  are  all  woody  forms,  but  they  may 
be  trailing  or  strangling  shrubs,  gigantic  trees,  or  high- 
climbing  vines ;  and  their  leaves  may  be  needle-like,  broad, 
or  "  fern-like."  For  our  purpose  it  will  be  only  necessary 
to  define  the  two  most  prominent  groups. 

235.  Cycads. — Cycads  are  tropical,  fern-like  forms,  with 
large  branched  (compound)  leaves.  The  stem  is  either  a 
columnar  shaft  crowned  with  a  rosette  of  great  branching 
leaves,  with  the  general  habit  of  tree-ferns  and  palms  (Figs. 
16,  316) ;  or  they  are  like  great  tubers,  crowned  in  the 
same  way.  In  ancient  times  (the  Mesozoic)  they  were  very 
abundant,  forming  a  conspicuous  feature  of  the  vegeta- 
tion, but  now  they  are  represented  only  by  about  eighty 
forms  scattered  through  both  the  oriental  and  occidental 
tropics. 


i:   V 


356 


PLANT   STUDIES 


236.  Conifers. — This  is  the  great  modern  Gymnosperm 
group,  and  is  characteristic  of  the  temperate  regions,  where 
it  forms  great  forests.  Some  of  the  forms  are  widely  dis- 
tributed, as  the  great  genus  of  pines  {Pinus)  (Fig.  57), 
while  some  are  now  very  much  restricted,  although  for- 
merly very  widely  distrib- 
uted, as  the  gigantic  red- 
woods (Sequoia)  of  the 
Pacific  slope.  The  habit  of 
the  body  is  quite  charac- 
teristic, a  central  shaft  ex- 
tending continuously  to  the 
very  top,  while  the  lateral 
branches  spread  horizontal- 
ly, with  diminishing  length 
to  the  top,  forming  a  coni- 
cal outline  (Figs.  56,  57). 
This  habit  of  firs,  pines, 
etc.,  gives  them  an  appear- 
ance very  distinct  from  that 
of  other  trees. 

Another  peculiar  feature 
is  furnished  by  the  char- 
acteristic "  needle-leaves," 
which  seem  to  be  poorly 
adapted  for  foliage.  These 
leaves  have  small  spread  of 
surface  and  very  heavy  pro- 
tecting walls,  and  show  adap- 
tation for  enduring  hard 
conditions  (Fig.  308).  As 
they  have  no  regular  period  of  falling,  the  trees  are  always 
clothed  with  them,  and  have  been  called  "  evergreens." 
There  are  some  notable  exceptions  to  this,  however,  as  in 
the  case  of  the  common  larch  or  tamarack,  which  sheds  its 
leaves  every  season  (Fig.  56). 


Fig.  317.  Arbor-vitae  (Thvja),  showing  a 
branch  with  scaly  overlapping  leaves, 
and  some  carpellate  cones  (strobili).— 
After  EicHLER. 


Fig.  31*.  The  common  juniper  (Juniperus  community,  the  branch  to  the  left  bearing 
staminatc  strobili;  that  to  the  right  bearing  staminate  strobili  above  and  carpel- 
lato  i^trobili  below,  which  latter  have  matured  into  the  fleshy,  berry-like  fruit. 
— After  Berg  and  Schmidt. 


CHAPTEE   XXIV 

SPERMATOPHYTES:  ANGIOSPERMS 

237.  Summary  of  Gymnosperms. — Before  beginning  An- 
giosperms  it  is  well  to  state  clearly  the  characters  of  Gym- 
nosperms which  have  set  them  apart  as  a  distinct  group  of 
Spermatophytes,  and  which  serve  to  contrast  them  with 
Angiosperms. 

(1)  The  microspore  (pollen-grain)  by  wind-pollination 
is  brought  into  contact  with  the  megasporangium  (ovule), 
and  there  develops  the  pollen-tube,  which  penetrates  the 
nucellus.  This  contact  between  pollen  and  ovule  implies 
an  exposed  or  naked  ovule  and  hence  seed,  and  therefore 
the  name  "  Gymnosperm." 

(2)  The  female  gametophyte  (endosperm)  is  well  organ- 
ized before  fertilization. 

(3)  The  female  gametophyte  produces  archegonia. 

238.  General  characters  of  Angiosperms. — This  is  the  great- 
est group  of  plants,  both  in  numbers  and  importance,  being 
estimated  to  contain  about  100,000  species,  and  forming 
the  most  conspicuous  part  of  the  vegetation  of  the  earth. 
It  is  essentially  a  modern  group,  replacing  the  Gymnosperms 
which  were  formerly  the  dominant  Seed-plants,  and  in  the 
variety  of  their  display  exceeding  all  other  groups.  The 
name  of  the  group  is  suggested  by  the  fact  that  the  seeds 
are  inclosed  in  a  seed  case,  in  contrast  with  the  exposed 
seeds  of  the  Gymnosperms. 

These  are  also  the  true  flowering  plants,  and  the  ap- 
pearance  of  true   flowers  means  the  development  of  an 
358 


SPERMATOPIIYTES  :    ANGIOSPEKMS 


359 


elaborate  symbiotic  relation  between  flowers  and  insects, 
through  wliich  pollination  is  secured.  In  Angiosperms, 
therefore,  the  wind  is  abandoned  as  an  agent  of  pollen 
transfer  and  insects  are  used ;  and  in  passing  from  Gym- 
nosperms  to  Angiosperms  one  passes  from  anemophilous  to 
entomophilous  ('•'insect-loving")  plants.  This  does  not 
mean  that  all  Angiosperms  are  entomophilous,  for  some  are 
still  wind-pollinated,  but  that  the  group  is  prevailingly  ento- 
mophilous. This  fact,  more  than  anything  else,  has  re- 
sulted in  a  vast  variety  in  the  structure  of  flowers,  so  char- 
acteristic of  the  group. 

239.  The  plant  body. — This  of  course  is  a  sporophyte, 
the  gametophytes  being  minute  and  concealed,  as  in  Gym- 
nosperms.  The  sporophyte  represents  the  greatest  possible 
variety  in  habit,  size,  and  duration,  from  minute  floating 
forms  to  gigantic  trees  ;  herbs,  shrubs,  trees ;  erect,  pros- 
trate, climbing  ;  aquatic,  terrestrial,  epiphytic  ;  from  a  few 
days  to  centuries  in  duration. 

Eoots,  stems,  and  leaves  are  more  elaborate  and  vari- 
ously organized  for  work  than  in  other  groups,  and  the 
whole  structure  represents  the  high- 
est organization  the  plant  body  has 
attained.  As  in  the  Gymnosperms, 
the  leaf  is  the  most  variously  used 
organ,  showing  at  least  four  distinct 
modifications  :  (1)  foliage  leaves,  (2) 
scales,  (3)  sporophylls,  and  (4)  floral 
leaves.  The  first  three  are  present 
in  Gymnosperms,  and  even  in  Pteri- 
dophytes,  but  floral  leaves  are  pecul- 
iar to  Angiosperms,  making  the  true 
flower,  and  being  associated  with  en- 
tomophily. 

240.  Microsporophylls. — The  micro- 
sporopliyll  of  Angiosperms  is  more 
definitely  known  as  a  *'  stamen  "  than 


Fig.  3in.  Stamens  of  hen- 
bane (HyoscyaTtiun) :  A, 
front  view,  showing  fila- 
ment {/)  and  anther  (/)); 
a,  back  view,  showing 
the  connective  (c)  be- 
tween the  pollen-sacs. 
—After  ScHiMPEB. 

24 


360 


PLANT   STUDIES 


that  of  Gymnosperms,  and  has  lost  any  semblance  to  a  leaf. 
It  consists  of  a  stalk-like  portion,  the  filament^  and  a 
sporangia-bearing  portion,  the  anther  (Figs.  319,  321,  A). 


Fig.  320.  Cross-section  of  anther  of  thorn  apple  (Datura),  showing  the  four  imbedded 
sporangia  (a,  p)  containing  microspores;  the  pair  on  each  side  will  merge  and 
dehisce  along  the  depression  between  them  for  the  discharge  of  pollen. — After 
Frank. 

The  filament  may  be  long  or  short,  slender  or  broad,  or 
variously  modified,  or  even  wanting.  The  anther  is  simply 
the  region  of  the  sporophyll  which  bears  sporangia,  and  is 


\v//i 


Fig.  321.  Diagrammatic  cross-sections  of  anthers:  A,  younger  stage,  showing  the 
four  imbedded  sporangia,  the  contents  of  two  removed,  but  the  other  two  con- 
taining pollen  mother  cells  {pm)  surrounded  by  the  tapetum  (t)\  B,  an  older  stage, 
in  which  the  microspores  (pollen  grains)  are  mature,  and  the  pair  of  sporangia  on 
each  side  are  merging  together  to  form  a  single  pollen-sac  with  longitudinal 
dehiscence.— After  Baillon  and  Luerssen. 

therefore  a  composite  of  sporophyll  and  sporangia  and  is 
often  of  uncertain  limitation.  Such  a  term  is  convenient, 
but  is  not  exact  or  scientific. 


SPERMATOPHYTES  :    ANGIOSPERMS 


361 


If  a  young  anther  be  sectioned  transversely  four  sporan- 
gia will  be  found  imbedded  beneath  the  epidermis,  a  pair 
on  each  side  of  the  axis  (Figs.  320,  321).  When  they  reach 
maturity,  the  paired  sporangia  on  each  side  usually  merge  to- 
gether, forming  two  spore-containing  cavities  (Fig.  321,  B). 
These  are  generally  called  ^'  pollen-sacs,"  and  each  anther  is 
said  to  consist  of  two  pollen-sacs,  although  each  sac  is  made 
up  of  two  merged  sporangia,  and  is  not  the  equivalent  of  the 
pollen-sac  in  Gymnosperms,  which  is  a  single  sporangium. 


Fig.  322.  Various  forms  of  stamens:  .-1,  from  Solatium,  showing  dehiscence  by 
terminal  pores;  B,  from  Arinitus,  showing  anthers  with  terminal  pores  and 
"horns";  C,  from  Berberis ;  D,  from  Atherospenna,  showing  dehiscence  by 
uplifted  valves;  E,  from  Aquilegia,  showing  longitudinal  dehiscence  ;  F,  from 
Popoina.  showing  pollen-sacs  near  the  middle  of  the  stamen.— After  Engler 
and  Prantl. 


362 


PLANT   STUDIES 


Fig.  323.  Cross  -  section  of 
anther  of  a  lily  {Butomus), 
showing  the  separating  walls 
between  the  members  of  each 
pair  of  sporangia  broken 
down  at  z,  forming  a  con- 
tinuous cavity  (pollen-sac) 
which  opens  by  a  longitudi- 
nal slit.— After  Sachs. 


The  opening  of  tlie  pollen-sac  to  discharge  its  pollen- 
grains  (microspores)  is  called  dehiscence^  which  means  "  a 
splitting  open,"  and  the  methods  of 
dehiscence  are  various  (Fig.  322). 
By  far  the  most  common  method 
is  for  the  wall  of  each  sac  to  split 
lengthwise  (Fig.  323),  which  is 
called  longitudinal  dehiscence;  an- 
other is  for  each  sac  to  open  by  a 
terminal  pore  (Fig.  322),  in  which 
case  it  may  be  prolonged  above  into 
a  tube. 

241.     Megasporophylls.  —  These 
are  the  so-called  "  carpels  "  of  Seed- 
plants,   and    in   Angiosperms   they 
are  organized  in  various  ways,  but 
always  so  as  to  inclose  the  mega- 
sporangia  (ovules).     In  the  simplest 
cases  each  carpel  is  independent  (Fig.  324,  A)^  and  is  dif- 
ferentiated into  three  regions:  (1)  a  hollow  bulbous  base, 
which  contains  the 

ovules  and  is  the  (I    w\ 

realseedcase,  r»     W   vn 

known  as  the 
ovary ;  (2)  sur- 
mounting this  is  a 
slender  more  or  less 
elongated  jorocess, 
the  style;  and  (3) 
usually  at  or  near 
the  apex  of  the  style 
a  special  receptive 
surface  for  the  pol- 
len, the  stigma. 

In  other   cases 
several  carpels  to- 


B 

Fig.  324.     Types   of  pistils 


A,  three  simple  pistils 


(apocarpous),  each  showing  ovary  and  style  tipped 
with  stigma ;  B,  a  compound  pistil  (syncarpous), 
showing  ovary  (/),  separate  styles  (g),  and  stigmas 
(«) ;  C,  a  compound  pistil  (syncarpous),  showing 
ovary  (/),  single  style  (g),  and  stigma  (n).— After 
Berg  and  Schmidt. 


SPEKMATOPIIYTES :    ANGIOSPERMS 


363 


gether  form  a  common  ovary,  while  the  styles  may  also 
combine  to  form  one  style  (Fig.  '624:,  C),  or  they  may  remain 
more  or  less  distinct  (Fig.  324,  B).  Such  an  ovary  may 
contain  a  single  chamber,  as  if  the  carpels  had  united  edge 
to  edge  (Fig.  325,  A) ;  or  it  may  contain  as  many  chambers 
as  there  are  constituent  carpels  (Fig.  325,  B),  as  though 
each  carpel  had  formed  its  own  ovary  before  coalescence. 
In  ordinary  phrase  an  ovary  is  '  either  "  one-celled "  or 
"  several-celled,"  but  as  the  word  "  cell "  has  a  very  differ- 
ent application,  the  ovary  chamber  had  better  be  called  a 
loculus,  meaning  "a  compartment."     Ovaries, 


A  li  C 

Fig.  325.  Diagrammatic  sections  of  ovaries :  A,  cross-section  of  an  ovary  with  one 
loculus  and  three  carpels,  the  three  sets  of  ovules  said  to  be  attached  to  the  wall 
(parietal) ;  B,  cross-section  of  an  ovary  with  three  loculi  and  three  carpels,  the 
ovules  being  in  the  center  (central) ;  C,  longitudinal  section  showing  ovules 
attached  to  free  axis  (free  central).— After  Schimper. 


therefore,  may  have  one  loculus  or  several  loculi.  Where 
there  are  several  loculi  each  one  usually  represents  a  con- 
stitutent  carpel  (Fig.  325,  B) ;  where  there  is  one  loculus 
the  ovary  may  comprise  one  carpel  (Fig.  324,  A),  or  several 
(Fig.  325,  A). 

There  is  a  very  convenient  but  not  a  scientific  word, 
which  stands  for  any  organization  of  the  ovary  and  the 
accompanying  parts,  and  that  is  pistil.  A  pistil  may  be 
one  carpel  (Fig.  324,  A),  or  it  may  be  several  carpels  or- 
ganized together  (Fig.  324,  B,  C),  the  former  case  being  a 
simple  pistil,  the  latter  a  compound  pistil.     In  other  words, 


364  PLANT   STUDIES 

any  organization  of  carpels  which  appears  as  a  single  organ 

with  one  ovary  is  a  pistil. 

The  ovules  (megasporangia)  are  developed  within  the 

ovary  (Fig.  325)  either  from  the  carpel  wall,  when  they  are 
foliar,  or  from  the  stem  axis  which  ends 
within  the  ovary,  when  they  are  cauline 
(see  §  89).  They  are  similar  in  struc- 
ture to  those  of  Gymnosperms,  with  in- 
tegument and  micropyle,  nucellus,  and 
embryo-sac  (megaspore),  except  that 
there  are  often  two  integuments,  an 
outer  and  an  inner  (Fig.  326). 

Fig.  326.  A  diagrammatic  242.  Modifications  of  the  flower. — In 

section  of  an  ovule  of     general,  the  flower  may  be  resrarded  as 

Angiospermg,    showing      °  j-n    j    x.  t     r.         ■  in 

outer  integument  {ai),     a  modified  branch  bearing  sporophylls 
inner  integument  («),     ^nd   usually  floral   Icavcs.      Its   rcprc- 

micropyle  (m),  nucellus  ±1,      r,^      -  j       \     i.  j 

(k),  and  embryo-sac  or     scutativc  amoug  the  rtcridophytes  and 
megaspore  (m).-After     Gymuospemis  is  the  strobilus,  which 
has  sporophylls  but  not  floral  leaves. 
In  Angiosperms  it  begins  in  a  simple  and  somewhat  indefi- 
nite way,  gradually  becomes  more  complex,  until  finally  it 
appears  as  an  elaborate  and  very  efiicient  structure. 

The  evolution  of  the  flower  has  proceeded  along  many 
lines,  and  has  resulted  in  great  diversity  of  structure.  These 
diversities  are  largely  used  in  the  classification  of  Angio- 
sperms, as  it  is  supposed  that  near  relatives  are  indicated 
by  similar  floral  structures,  as  well  as  by  other  features. 
Some  of  the  lines  of  evolution  may  be  indicated  as  fol- 
lows : 

1.  Fro7n  naked  flowers  to  those  ivitli  distinct  calyx  and 
corolla. — In  the  simplest  flowers  floral  leaves  do  not  appear, 
and  the  flower  is  represented  only  by  the  sporophylls. 
When  the  floral  leaves  first  appear  they  are  inconspicuous, 
scale-like  bodies.  In  higher  forms  they  become  more  promi- 
nent, but  are  still  all  alike.  At  last  the  floral  leaves  become 
differentiated,  the  outer  set  (calyx)  remaining  scale-like  or 


SPERMATOPHYTES:    ATs^GIOSPERMS  3^5 

like  small  foliage  leaves,  and  the  inner  set  (corolla)  becom- 
ing more  delicate  in  texture,  larger,  and  generally  brightly 
colored  (Fig.  71). 

2.  From  spiral  to  cyclic  flowers. — In  the  simplest  flowers 
the  sporophylls  and  floral  leaves  (if  any)  are  distributed 
about  an  elongated  axis  in  a  spiral,  like  a  succession  of 
leaves.  As  this  axis  is  elongated  and  capable  of  continued 
growth,  an  indefinite  number  of  each  floral  organ  may  ap- 
pear. The  spiral  arrangement  and  indefinite  numbers, 
therefore,  are  regarded  as  primitive  characters. 

In  higher  forms  the  axis  becomes  shorter,  the  spiral 
closer,  until  finally  the  sets  of  organs  seem  to  be  thrown 
into  rosettes  or  cycles.  These  cycles  may  not  appear  in  all 
the  organs  of  a  flower,  but  finally,  in  the  highest  forms,  all 
the  fioral  organs  are  in  definite  cycles.  All  through  this 
evolution  from  the  spiral  to  the  cyclic  arrangement  there 
is  constantly  appearing  a  tendency  to  "  settle  down  "  to 
certain  definite  numbers,  and  when  the  complete  cyclic 
arrangement  is  finally  established  these  numbers  are  estab- 
lished, and  they  become  characteristic  of  great  groups. 
For  example,  in  the  cyclic  Monocotyledons  there  are  nearly 
always  just  three  organs  in  each  cycle,  while  in  the  cyclic 
Dicotyledons  the  number  five  prevails. 

3.  From  hypogynous  to  epigynous  floioers. — In  the  sim- 
pler fiowers  the  sepals,  petals,  and  stamens  arise  from  be- 
neath the  ovary  or  ovaries  (Fig.  72,  i),  and  as  in  such  cases 
the  ovary  may  be  seen  distinctly  above  the  origin  (inser- 
tion) of  the  other  parts,  such  a  flower  is  often  said  to  have 
a  "superior  ovary,"  or  to  be  hypogynous.,  meaning  in  effect 
"  under  the  ovary,"  referring  to  the  fact  that  the  insertion 
of  the  other  parts  is  under  the  ovary. 

There  is  a  distinct  tendency,  however,  for  the  insertion 
of  the  outer  parts  to  be  carried  higher  up,  until  finally  it  is 
above  the  ovary,  and  sepals,  petals,  and  stamens  seem  to 
arise  from  the  top  of  the  ovary  (Fig.  72, «?),  such  a  flower 
being  epigynous.     In  such  cases  the  ovary  does  not  appear 


3^6  PLANT   STUDIES 

within  the  flower,  but  below  it  (Fig.  132),  and  the  flower 
is  often  said  to  have  an  "  inferior  ovary." 

4.  From  apocarpous  to  syncarpous  flowers.  —  In  the 
simpler  flowers  the  carpels  are  entirely  distinct,  each  car- 
pel organizing  a  simple  pistil,  a  single  flower  containing  as 
many  pistils  as  there  are  carpels  (Fig.  324,  A).  Such  a 
flower  is  said  to  be  apocarpous.,  meaning  "  carpels  separate." 
There  is  a  very  strong  tendency,  however,  for  the  carpels  of 
a  flower  to  organize  together  and  to  form  a  single  com- 
pound pistil  (Fig.  324,  B^  C),  such  a  flower  being  called 
syncarpous^  meaning  "carpels  together." 

5.  From  poly  pet  alous  to  sympetalous  flowers. — While  the 
petals  are  entirely  distinct  from  one  another  in  the  lower 
forms,  a  condition  described  as  poly  pet  alous.,  in  the  highest 
Angiosperms  they  are  coalescent,  the  corolla  thus  becoming 
a  more  or  less  tubular  organ  (Figs.  73,  74).  Such  flowers 
are  said  to  be  sympetalous,  meaning  "  petals  united." 

6.  From  regular  to  irregular  flowers. — In  the  simplest 
flowers  all  the  members  of  one  set  are  alike,  and  the  flower 
is  said  to  be  regular  (Fig.  74,  a,  h).  In  certain  lines  of 
advance,  however,  there  is  a  tendency  for  some  of  the  mem- 
bers of  a  single  set,  particularly  the  petal  set,  to  become 
unlike.  For  example,  in  the  common  violet  one  of  the 
petals  develops  a  spur ;  while  in  the  sweet  pea  the  petals 
are  remarkably  unlike.  Such  flowers  are  said  to  be  irregu- 
lar (Fig.  74,  c,  ^,  e),  and  as  a  rule  irregularity  is  associated 
with  adaptations  for  insect  pollination. 

These  various  lines  appear  in  all  stages  of  advancement 
in  different  flowers,  so  that  it  would  be  impossible  to  deter- 
mine the  relative  rank  in  all  cases.  However,  if  a  flower  is 
naked,  with  indefinite  numbers,  hypogynous,  and  apocar- 
pous, it  would  rank  very  low ;  but  if  it  has  a  calyx  and 
corolla,  is  completely  cyclic,  epigynous,  syncarpous,  sym- 
petalous, and  irregular,  it  would  rank  very  high. 

243.  The  gametophytes. — As  in  the  case  of  the  Gymno- 
sperms,  the  gametophytes  of  Angiosperms  are  exceedingly 


SPERMATOPIIYTES  :    ANGIOSPERMS 


367 


simple,  being  developed  entirely  within  the  spores  which 
produce  them. 

The  male  gametophyte  is  represented  by  a  few  cells  which 
appear  within  the  pollen  grain,  two  of  which  are  male  cells. 

When  pollination 
occurs,  and  the  pollen 
has  been  transferred 
from  the  pollen-sacs  to 
the  stigma,  it  is  de- 
tained by  the  minute 
papilla3  of  the  stig- 
matic  surface,  which 
also  excretes  a  sweet- 
ish sticky  fluid.  This 
fluid  is  a  nutrient  so- 
lution for  the  micro- 
spores, which  begin  to 
put  out  their  tubes.  A 
13ollen-t.ube  penetrates 
through  the  stigmatic 
surface,  enters  among 
the  tissues  of  the  style, 
which  is  sometimes 
very  long,  slowly  or 
rapidly  traverses  the 
length  of  the  style  sup- 
plied with  food  by  its 
cells  but  not  penetrat- 
ing them,  enters  the 
cavity  of  the  ovary, 
passes  througli  the 
micropyle  of  an  ovule, 
penetrates  the  tissues 
of  the  nucellus  (if  any), 
and  finally  reaches  and  pierces  the  wall  of  the  embryo-sac, 
within  which  is  the  egg  awaiting  fertilization  (Fig.  327). 


Fig.  327.  Diasrram  of  a  longitudinal  section  through 
a  carpel,  to  illustrate  fertilization  with  all  parts 
in  place  :  .«.  stigma  ;  g,  style  ;  o.  ovary  ;  ai,  ii, 
outer  and  inner  integuments  ;  n,  base  of  nucel- 
lus ;  /.  funiculus  ;  b,  antipodal  cells  ;  c,  endo- 
sperm nucleus  ;  k.  egg  and  one  synergid  ;  p,  pol- 
len-tube, having  grown  from  stigma  and  passed 
through  the  microjjylo   (»i)  to  the  egg.— After 

LUEUSSEN. 


368 


PLANT   STUDIES 


The  female  gametophyte  develops  within  the  embryo- 
sac,  and  consists  at  first  of  seven  independent  cells,  one 
of  which  is  the  egg,  no  archegonium  being  formed.     The 


Fig.  328.  Development  of  embryo  of  shepherd's  purse  (Capsella),  a  Dicotyledon; 
beginning  with  /,  the  youngest  stage,  and  following  the  sequence  to  VI,  the  old- 
est stage,  V  represents  the  suspensor,  c  the  cotyledons,  s  the  stem-tip,  ^v  the  root, 
h  the  root-cap.  Note  the  root-tip  at  one  end  of  the  axis  and  the  stem-tip  at  the 
other  between  the  cotyledons. — After  Hanstein. 


egg  is  in  the  end  of  the  sac  nearest  the  micropyle,  in  the 
most  convenient  position  for  the  entering  tnbe.  AVhen  the 
tip  of  the  pollen-tube  enters  the  sac  it  discharges  the  two 
male  cells.  One  of  these  unites  with  the  egg  and  forms 
the  oospore,  which  germinates  and  forms  the  embryo.  The 
other  male  cell  unites  with  one  of  the  other  free  cells  of 
the  female  gametophyte  and  forms  the  "  endosperm  cell," 


SPERMATOPIIYTES :    ANGIOSPERMS 


369 


which  divides  and  begins  the  formation  of  the  endosperm, 
a  tissue  that  feeds  the  embryo  and  is  often  the  nutritive 
part  of  seeds.  In  Angiosperms,  therefore,  there  are  two 
simultaneous  acts  of  fertilization,  one  starting  the  embryo, 
the  other  the  endosperm,  and  hence  in  this  group  "  double 
fertilization ''  is  said  to  occur. 

244.  The  embryo. — When  the  oospore  germinates,  a  more 
or  less  distinct  suspensor  is  usually  formed,  but  never  so 
prominent  as  in  Gymnosperms ;  and  at  the  end  of  the 
suspensor  the  embryo  is  developed, 
which,  when  completed,  is  more  or 
less  surrounded  by  nourishing  en- 
dosperm, or  has  stored  wp  within 
its  seed-leaves  an  abundant  food 
supply. 

The  two  groups  of  Angiosperms 
differ  widely  in  the  structure  of  the 
embryo.  In  Monocotyledons  the  axis 
of  the  embryo  develops  the  root-tip  at 
one  end  and  the  "  seed-leaf  "  (cotyle- 
don) at  the  other,  the  stem-tip  arising 
from  the  side  of  the  axis  as  a  lateral 
member  (Fig.  329). 

In  Dicotyledons  the  axis  of  the 
embryo  develops  the  root-tip  at  one 
end  and  the  stem-tip  at  the  other, 
the  cotyledons  (usually  two)  appear- 
ing as  a  pair  of  opposite  lateral 
members  on  either  side  of  the  stem- 
tip  (Fig.  328).  As  the  cotyledons 
are  lateral  members  their  number 
may  vary. 

The  axis  of  the  embryo  between  the  root-tip  and  the 
cotyledons  is  called  the  hjipocotyl  (Figs.  143,  315,  331),  which 
means  "  under  the  cotyledon,"  a  region  which  shows  pecul- 
iar activity  in  connection  with  the  escape  of  the  embryo 


Fig.  329.  Young  embryo  of 
water  plantain  (Alisma),  a 
Monocotyledon,  the  root 
being  organized  at  one 
end  (next  the  suspensor), 
the  single  cotyledon  (O 
at  the  other,  and  the  stem- 
tip  arising  from  a  lateral 
notch  {v).  —  After  Han- 
stein. 


370 


PLANT   STUDIES 


from  the  seed.  Formerly  it  was  called  either  cauUde  or 
radicle.  In  Dicotyledons  the  stem-tip  between  the  coty- 
ledons often  organizes  the  rudiments  of  subsequent  leaves, 
forming  a  little  bud  which  is  called  the  plumule. 

Embryos  differ  much  as  to  completeness  of  their  devel- 
opment within  the  seed.  In  some  plants,  especially  those 
which  are  parasitic  or  saprophytic,  the  embryo  is  merely  a 
small  mass  of  cells,  without  any  organization  of  root,  stem, 
or  leaf.  In  many  cases  the  embryo  becomes  highly  devel- 
oped, the  endosperm  being  used  up  and  the  cotyledons 
stuffed  with  food  material,  the  plumule  containing  several 
well-organized  young  leaves,  and  the  embryo  completely 
filling  the  seed  cavity.  The  common  bean  is  a  good  illus- 
tration of  this  last  case,  the  whole  seed  within  the  integu- 
ment consisting  of  the  two  large,  fleshy  cotyledons,  between 
which  lie  the  hypocotyl  and  a  plumule  of  several  leaves. 

245.  The  seed. — As  in  Gymnosperms,  while  the  processes 
above  described  are  taking  place  within  the  ovule,  the  in- 
tegument or  integuments  are  becoming  transformed  into 
the  testa  (Fig.  330).     When  this  hard  coat  is  fully  devel- 


FiG.  330.  The  two  figures  to  the  left  are  seeds  of  violet,  one  showing  the  black,  hard 
testa,  the  other  being  sectioned  and  showing  testa,  endosperm,  and  imbedded 
embryo  ;  the  figure  to  the  right  is  a  section  of  a  pepper  fruit  (Piper),  showing 
modified  ovary  wall  (pc),  seed  testa  (sc),  nucellus  tissue  {p),  endosperm  (en),  and 
embryo  (^m).— After  Atkinson. 

oped,  the  activities  within  cease,  and  the  whole  structure 
passes  into  that  condition  of  suspended  animation  which  is 
so  little  understood,  and  which  may  continue  for  a  long 
time. 


SPERMATOPHYTES  :    ANGIOSPEKMS 


371 


The  testa  is  variously  developed  in  seeds,  sometimes 
being  smooth  and  glistening,  sometimes  pitted,  sometimes 
rough  with  warts  or  ridges.  Sometimes  prominent  append- 
ages are  produced  which  assist  in  seed-dispersal,  as  the 
wings  in  Catalpa  or  Bignonia  (Fig.  115),  or  the  tufts  of 
hair  on  the  seeds  of  mi]kweed,  cotton,  or  fireweed. 

216.  The  fruit— The  effect  of  fertilization  is  felt  beyond 
the  boundaries  of  the  ovule,  which  forms  the  seed.  The 
ovary  is  also  involved,  and  becomes  more  or  less  modified. 
It  enlarges  more  or  less,  sometimes  becoming  remarkably 
enlarged.  It  also  changes  in  structure,  often  becoming 
hard  or  parchment-like.  In  case  it  contains  several  or 
numerous  seeds,  it  is  organized  to  open  in  some  way  and 
discharge  them,  as  in  the  ordinary  i^ocls  and  capsules  (Fig. 
122).  In  case  there  is  but  one  seed,  the  modified  ovary 
wall  may  invest  it  as  closely  as  another  integument,  and  a 
seed-like  fruit  is  the  result — a  fruit  which  never  opens  and 
is  practically  a  seed.  Such  a  fruit  is  known  as  an  akene^ 
and  is  very  characteristic  of  the  greatest  Angiosperm  family, 
the  Compositae,  to  which  sunflowers,  asters,  golden-rods, 
daisies,  thistles,  dandelions,  etc.,  belong.  Dry  fruits  which 
do  not  open  to  discharge  the  seed  often  bear  appendages 
to  aid  in  dispersal  by  wind  (Figs.  116,  117),  or  by  animals 
(Fig.  129). 

Capsules,  pods,  and  akenes  are  said  to  be  dry  fruits,  but 
in  many  cases  fruits  ripen  fleshy.  In  the  peach,  plum, 
cherry,  and  all  ordinary  "  stone  fruits,"  the  modified  ovary 
wall  organizes  two  layers,  the  inner  being  very  hard,  form- 
ing the  "stone,"  the  outer  being  pulpy,  or  variously  modi- 
fied (Fig.  330).  In  the  true  berries,  as  the  grape,  currant, 
tomato,  etc.,  the  whole  ovary  becomes  a  thin-skinned  pulpy 
mass  in  which  the  seeds  are  imbedded. 

In  some  cases  the  effect  of  fertilization  in  changing 
structure  is  felt  beyond  the  ovary.  In  the  apple,  pear, 
quince,  and  such  fruits,  the  pulpy  part  is  the  modified 
calyx  (one  of  the  floral  leaves),  the  ovary  and  its  contained 


372 


PLANT   STUDIES 


seeds  being  represented  by  the  "  core."  In  other  cases,  the 
end  of  the  stem  bearing  the  ovaries  (receptacle)  becomes 
enlarged  and  pulpy,  as  in  the  strawberry.  This  effect 
sometimes  involves  even  more  than  the  parts  of  a  single 
flower,  a  whole  flower-cluster,  with  its  axis  and  bracts,  be- 
coming an  enlarged  pulpy  mass,  as  in  the  pineapple. 

The  term  "  fruit,"  therefore,  is  a  very  indefinite  one,  so 
far  as  the  structures  it  includes  are  concerned. 

247.  The  germination  of  the  seed, — It  is  wrong  to  apply 
the  term  "  germination  "  to  the  renewal  of  activity  by  the 
young  plantlet  within  the  seed,  as  has  been  shown  before 
(page  354),  but  in  the  absence  of  a  better  word  it  will  be 
used.  This  "  awakening  of  the  seed  "  is  a  phenomenon  so 
easily  observed  that  it  can  hardly  escape  the  attention  of 
any  one. 

Just  how  long  different  seeds  may  retain  their  vitality — 
that  is,  live  in  a  state  of  suspended  animation — is  not  very 
definitely  known.  Some  seeds  have  germinated  after  hav- 
ing remained  in  a  dried-up  condition  for  many  years,  but 
such  stories  as  that  wheat  taken  from  the  wrappings  of 
Egyptian  mummies  has  been  made  to  germinate  are  myths. 

If  the  structures  of  the  seed  are  normal,  its  germination 
will  follow  its  exposure  to  certain  conditions,  prominent 
among  which  are  water,  heat,  and  oxygen.  Seeds  vary  in 
the  amount  of  water  and  heat  absolutely  needed,  but  for 
terrestrial  plants  all  the  suitable  conditions  are  supplied 
by  burial  in  loose,  moist  soil,  at  the  temperatures  which 
prevail  during  the  growing  season. 

This  so-called  germination  is  merely  a  rencAval  of  the 
growth  of  the  embryo,  which  results  in  freeing  it  from  the 
seed  coats,  and  in  enabling  it  to  establish  itself  for  inde- 
pendent living.  All  the  conditions  for  growth  are  present, 
namely,  food  material^  stored  within  the  seed,  most  com- 
monly as  starch  or  oil ;  oxygen^  to  be  used  in  respiration ; 
water^  to  put  the  cells  in  proper  condition  for  work,  and 
to  act  as  an  agent  of  transfer;  and   a   suitable  tempera- 


SPERMATOPHYTES:    ANGIOSPEKMS  373 

ture^  necessary  for  the  chemical  changes  about  to  be 
made. 

The  first  conspicuous  change  noted  in  the  seed  after  the 
absorption  of  water  is  the  softening  of  the  contents,  the 
solid  and  insoluble  starch,  if  that  be  the  form  of  the  food 
storage,  being  converted  by  a  process  of  digestion  into 
soluble  sugar,  ready  for  transfer.  The  digestive  substance 
is  known  as  enzyme^  and  the  most  abundant  enzyme  in 
seeds  is  diastase^  which  has  the  power  of  transforming 
starch  into  a  sugar.  Accompanying  these  changes  there  is 
to  be  noted  a  marked  evolution  of  heat,  so  that  if  a  large 
mass  of  seeds  is  set  to  germinating,  as  in  the  process 
known  as  malting,  the  amount  of  heat  generated  may  be 
very  great. 

The  first  part  of  the  embryo  to  protrude  from  the  seed 
is  the  tij)  of  the  hypocotyl,  thrust  out  by  the  rapid  elonga- 
tion of  the  upper  part  of  the  hypocotyl  (Fig.  143,  B).  This 
protruding  and  rapidly  elongating  tip,  which  is  to  develop 
the  root,  now  rapidly  elongates  and  is  very  sensitive  to  the 
influence  of  gravity,  responding  by  developing  any  curva- 
ture necessary  to  reach  the  soil.  Penetrating  the  soil,  and 
beginning  to  put  out  lateral  branches,  it  secures  the  grip 
necessary  for  the  extrication  of  other  regions  of  the  em- 
bryo. 

After  some  anchorage  has  thus  been  obtained,  the  upper 
part  of  the  hypocotyl  again  begins  a  period  of  rapid  elonga- 
tion, which  results  in  the  development  of  a  curvature  known 
as  the  "  hypocotyl  arch  "  (Figs.  143,  (7,  and  143,  a).  In 
the  case  of  the  germinating  bean  this  arch  is  the  first  struc- 
ture to  appear  above  ground,  and  its  pull  upon  the  seed 
is  very  apt  to  bring  it  to  the  surface. 

Finally,  the  arch,  in  its  effort  to  straighten,  pulls  the 
cotyledons  out  of  the  seed-coats  and  with  tliem  tlie  stem 
tip,  the  axis  of  the  plant  straightens  up  (Fig.  143,  a),  the 
seed-leaves  and  sometimes  other  leaves  expand,  and  ger- 
mination   is  over ;  for  with  roots  in   the   soil,  and  green 


374 


PLANT   STDDIES 


leaves  expanded  to  the  air  and  sunlight,  the  plantlet  has 
become  independent  (Fig.  331). 

It  must  not  be  supposed  that  all  of  the  details  just 
given  apply  to  the  germination  of  all  seeds,  for  there  are 
certain  notable  variations.  For  ex- 
ample, in  the  pea  and  acorn  the 
cotyledons,  so  gorged  with  food  as 
to  have  lost  all  power  of  acting  as 
leaves,  are  never  extricated  from 
the  seed-coats,  but  the  stem  tip, 
which  lies  between  the  cotyledons, 
is  pushed  out  by  the  elongation  of 
the  cotyledons  at  base  into  short  or 
sometimes  long  stalks.  In  the  ce- 
reals, as  corn,  wheat,  etc.,  the  em- 
bryo lies  close  against  one  side  of 
the  seed,  so  that  it  is  completely 
exposed  by  the  splitting  of  the  thin 
skin  which  covers  it.  In  such  a 
case  the  cotyledon  is  never  un- 
folded, but  remains  as  an  absorbing 
organ,  while  the  root  extends  in 
one  direction,  and  the  stem,  with 
its  succession  of  unsheathing  leaves, 
develops  in  the  other. 

248.  Summary  from  Angiosperms. 
— At  the  beginning  of  this  chapter 
Fig.  331.  Seedling  of  hornbeam  (§  237)  the  characters  of  the  Gym- 
nosperms  were  summarized  which 
distinguished  them  from  Angio- 
sperms, whose  contrasting  charac- 
ters may  be  stated  as  follows  : 

(1)  The  microspore  (pollen- 
grain),  chiefly  by  insect  pollination, 
is  brought  into  contact  with  the  stigma,  which  is  a  recep- 
tive region  on  the   surface  of  the  carpel,   and  there  de- 


(Carpimis),  showing  pri- 
mary root  (hiv)  bearing  root- 
lets {sw)  upon  which  are 
numerous  root  hairs  (r),  hy- 
pocotyl  (h),  cotyledons  (c), 
young  stem  (e),  and  first  (1) 
and  second  {l')  true  leaves. 
— After  ScHiMPER. 


SPERMATOPHYTES:    ANGIOSPERMS  375 

velops  the  pollen-tube,  which  penetrates  the  style  to  reach 
the  ovary  cavity  which  contains  the  ovules  (megasporangia). 
The  impossibility  of  contact  between  pollen  and  ovule  im- 
plies inclosed  ovules  and  hence  seeds,  and  therefore  the 
name  "  Angiosperm." 

(2)  The  female  gametophyte  is  but  slightly  developed 
before  fertilization,  the  egg  appearing  very  early. 

(3)  The  female  gametophyte  produces  no  archegonia, 
but  a  single  naked  egg. 


25 


CHAPTER  XXV 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


249.  Contrasting  characters. — The  two  great  groups  of 
Angiosperms  are  quite  distinct,  and  there  is  usually  no  dif- 
ficulty in  recognizing  them.  The  monocotyledons  are 
usually  regarded  as  the  older  and  the  simpler  forms,  and 
are  represented  by  about  twenty  thousand  species.  The 
Dicotyledons  are  much  more  abundant  and  diversified,  con- 
taining about  eighty  thousand  species,  and  form  the  domi- 
nant vegetation  almost  everywhere. 
The  chief  contrasting  characters 
may  be  stated  as  follows  : 

Mo7iocotyledo7is.  —  (1)  Embryo 
with  terminal  cotyledon  and  lat- 
eral stem-tip.  This  character  is 
practically  without  exception. 

(2)  Vascular  bundles  of  stem 
scattered  (Fig.  332).  This  means 
that  there  is  no  annual  increase  in 
the  diameter  of  the  woody  stems, 
and  no  extensive  branching,  but 
to  this  there  are  some  exceptions. 

(3)  Leaf  veins  forming  a  closed 
system  (Fig.  333,  figure  to  left). 
As  a  rule  there  is  an  evident  set 

of  veins  which  run  approximately  parallel,  and  intricately 

branching  between  them  is  a  system  of  minute  veinlets  not 

readily  seen.     The  vein  system  does  not  end  freely  in  the 
376 


Fig.  332.  Section  of  stem  of 
corn,  showing  the  scattered 
bundles,  indicated  by  black 
dots  in  cross-section,  and  by 
lines  in  longitudinal  section. 
—From  "  Plant  Relations." 


MONOCUTYLEDUJSS   AJSD   DICUTVLEDUNS  377 

margin  of  the  leaf,  but  forms  a  ^^  closed  venation/'  so  that 
the  leaves  usually  have  an  even  (entire)  margin.     There 

are  some  notable  exceptions 
to  this  character. 

(4)   Cyclic  flowers  trim- 
erous.    The  "three-parted" 


Fig.  333.  Two  types  of  leaf  venation:  the  figure  to  the  left  is  from  Solomon's  seal, 
a  Monocotyledon,  and  shows  the  principal  veins  parallel,  the  very  minute  cross 
veinlets  being  invisible  to  the  naked  eye;  that  to  the  right  is  from  a  willow,  a 
Dicotyledon,  and  shows  netted  veins,  the  main  central  vein  (midrib)  sending  out 
a  series  of  parallel  branches,  which  are  connected  with  one  another  by  a  network 
of  veinlets.— After  Ettingshausen. 


flowers  of  cyclic  Monocotyledons  are  quite  characteristic, 
but  there  are  some  trimerous  Dicotyledons. 

Dicotyledons. — (1)  Embryo  witli  lateral  cotyledons  and 
terminal  stem-tip. 

(2)  Vascular  bundles  of  stem  forming  a  hollow  cylinder 
(Fig.  334,  iv).     This  means  an  annual  increase  in  the  diam- 


378 


PLANT   STUDIES 


¥iG.  334.  Section  across  a  young  twig  of 
box  elder,  showing  the  four  stem  regions: 
e,  epidermis,  represented  by  the  heavy 
bounding  line;  c,  cortex;  tv,  vascular  cyl- 
inder; 2h  pith.— From  "  Plant  Relations." 


eter  of  woody  stems  (Fig. 
335,  w),  and  a  possible 
increase  of  the  branch 
system  and  foliage  dis- 
play each  year. 

(3)  Leaf  veins  form- 
ing an  open  system  (Fig. 
333,  figure  to  right). 
The  network  of  smaller 
veinlets  between  the 
larger  veins  is  usually 
very  evident,  especially 
on  the  under  surface  of 
the  leaf,  suggesting  the 
name  ^^ net- veined '' 
leaves,  in  contrast  to  the  "  parallel-veined  "  leaves  of  Mono- 
cotyledons. The  vein  system  ends  freely  in  the  margin  of 
the  leaf,  forming  an  ^^open  venation."  In  consequence  of 
this,  although  the  leaf 

may  remain  entire,  it  .^^^^S^?^^^^>v,^^ 

very   commonly  be-  y^-^/^^^^mW  \ilfTm^.--:s'K^/// 

comes  toothed,  lobed, 
and  divided  in  various 
ways.  Two  main  types 
of  venation  may  be 
noted,  Avhich  influence 
the  form  of  leaves.  In 
one  case  a  single  very 
prominent  vein  (?'i^) 
runs  through  the  mid- 
dle of  the  blade,  and 
is  called  the  midrib. 
From  this  all  the  mi- 


nor veins  arise  as 
branches  (Fig.  336), 
and  such  a  leaf  is  said 


Fig.  335.  Section  across  a  twig  of  box  elder 
three  years  old,  showing  three  annual  rings, 
or  growth  rings,  in  the  vascular  cylinder;  the 
radiating  lines  (m)  which  cross  the  vascular 
region  {w)  represent  the  pith  rays,  the  princi- 
pal ones  extending  from  the  pith  to  the  cor- 
tex (c).— From  "  Plant  Relations." 


MONOCOTYLEDONS  AND  DICOTYLEDONS 


379 


to  be  pinnate  or  pmnately  veined,  and  inclines  to  elongated 
forms.  In  the  other  case  several  ribs  of  equal  j^rominence 
enter  the  blade  and  diverge  through  it  (Fig.  336).  Such 
a  leaf  is  palmate  or  palmately  veined,  and  inclines  to  broad 
forms. 

(4)  Cyclic  flowers  pentamerous  or  tetramerous.      The 
flowers  "in  fives"  are  greatly  in  the  majority,  but  some 


Fig.  336.   Leaves  showing  pinnate  and  palmate  branching;  the  one  to  the  left  is  from 
sumach,  that  to  the  right  from  buckeye. — Caldwell. 


very  prominent  families  have  flowers  ''in  fours."  There 
are  also  dicotyledonous  families  with  flowers  ''  in  threes," 
and  some  with  flowers  ''  in  twos." 

It  should  be  remembered  that  no  one  of  the  above  char- 
acters, unless  it  be  the  character  of  the  embryo,  should  be 
depended  upon  absolutely  to  distinguish  these  two  groups. 


380  PLANT  STUDIES 

It  is  the  combination  of  characters  which  determines  a 
group. 

250.  Monocotyledons. — In  the  Monocotyledons  about  forty 
families  are  recognized,  containing  numerous  genera,  and 
among  these  genera  the  twenty  thousand  species  are  dis- 
tributed. It  is  evident  that  it  will  be  impossible  to  con- 
sider such  a  vast  array  of  forms,  even  the  families  being  too 
numerous  to  mention. 

Prominent  among  the  families  are  the  aquatic  pond- 
weeds  of  various  kinds,  the  marshy  ground  cat-tails,  the 
grasses  and  sedges,  the  tropical  palms,  the  aroids,  the  lilies, 
and  the  orchids.  Of  these,  the  grasses  form  one  of  the 
largest  and  one  of  the  most  useful  groups  of  plants.  It  is 
world-wide  in  its  distribution,  and  is  remarkable  in  its  dis- 
play of  individuals,  often  growing  so  densely  over  large 
areas  as  to  form  a  close  turf.  If  the  grass-like  sedges 
be  associated  with  them  there  are  about  six  thousand 
species,  representing  nearly  one  third  of  the  Monocotyle- 
dons. Here  belong  the  various  cereals,  sugar-canes,  bam- 
boos, and  pasture  grasses,  all  of  them  immensely  useful 
plants. 

The  palms  and  the  aroids  each  number  about  one  thou- 
sand species,  and  are  conspicuous  members  of  tropical  vege- 
tation. 

In  temperate  regions,  however,  the  lilies  and  their  allies 
stand  as  the  best  representatives  of  Monocotyledons,  with 
their  usually  conspicuous  and  well-organized  flowers. 

In  number  of  species  the  orchids  form  the  greatest 
family  among  the  Monocotyledons,  the  species  being  vari- 
ously estimated  from  six  thousand  to  ten  thousand.  In 
display  of  individuals,  however,  the  orchids  are  not  to  be 
compared  with  the  grasses,  or  even  with  the  lilies,  for  in 
general  they  are  what  are  called  "rare  plants."  Orchids 
are  the  most  highly  developed  of  Monocotyledons,  and  their 
brilliant  coloration  and  bizarre  forms  are  associated  with 
marvellous  adaptations  for  insect  visitation. 


MONOCOTYLEDONS   AND  DICOTYLEDONS  381 

251.  Dicotyledons.— Dicotyledons  form  the  greatest  group 
of  plants  in  rank  and  in  numbers,  being  the  most  highly 
organized,  and  containing  about  eighty  thousand  species. 
They  represent  the  dominant  and  successful  vegetation  in 
all  regions,  and  are  especially  in  the  preponderance  in  tem- 
perate regions.  They  are  herbs,  shrubs,  and  trees,  of  every 
variety  of  size  and  habit,  and  the  rich  display  of  leaf  forms 
is  notably  conspicuous. 

Two  great  groups  of  Dicotyledons  are  recognized,  the 
ArchichlamydecB  and  the  SympetalcB.  In  the  former  there 
is  either  no  perianth  or  its  parts  are  separate  (polypeta- 
lous)  ;  in  the  latter  the  corolla  is  sympetalous.  The  Archi- 
chlamydeae  are  the  simpler  forms,  beginning  in  as  simple  a 
fashion  as  do  the  Monocotyledons;  while  the  Sympetalae 
are  evidently  derived  from  them  and  become  the  most 
highly  organized  of  all  plants.  The  two  groups  each  con- 
tain about  forty  thousand  species,  but  the  Archichlamydeae 
contain  about  one  hundred  and  sixty  families,  and  the 
Sympetalae  about  fifty. 

(1)  ArcMcUamydecB. — In  this  great  division  of  Dicoty- 
ledons are  such  groups  as  the  great  tree  alliance  which 
includes  poplars,  oaks,  hickories,  elms,  willows,  etc. ;  the 
buttercup  alliance,  which  includes  buttercups,  water-lilies, 
poppies,  mustards,  etc. ;  the  rose  family,  one  of  the  best 
known  and  most  useful  groups  of  the  temperate  regions ; 
the  pea  family,  by  far  the  greatest  family  of  the  Archi- 
chlamydeae, containing  about  seven  thousand  species ;  the 
parsley  family,  or  limbellifers,  containing  numerous  useful 
forms,  and  being  the  most  highly  organized  family  of  the 
Archichlamydeae. 

(2)  Sympet(d(B. — These  are  the  highest  and  the  most 
recent  Dicotyledons.  While  they  contain  numerous  shrubs 
and  trees  in  the  tropics,  they  are  by  no  means  such  a 
shrub  and  tree  group  in  the  temperate  regions  as  are  the 
Archichlamydeae.  The  flowers  are  constantly  cyclic,  the 
number   five   or   four   is    established,   and    the    corolla   is 


382  PLANT   STUDIES 

sympetalous,  the   stamens   usually  being  borne  upon  its 
tube. 

Among  the  numerous  families  the  following  are  promi- 
nent :  the  heaths,  mostly  shrubs  of  temperate  and  arctic  or 
alpine  regions  ;  the  convolvulus  alliance,  with  corolla  in  the 
form  of  conspicuous  tubes,  funnels,  trumpets,  etc. ;  the 
aromatic  mint  family,  with  more  than  ten  thousand  species, 
and  its  allies  the  nightshades,  the  figworts,  and  the  ver- 
benas ;  and,  last  and  highest,  the  family  of  composites,  the 
greatest  and  ranking  family  of  Angiosperms,  estimated  to 
contain  at  least  twelve  thousand  species,  more  than  one 
seventh  of  all  known  Dicotyledons,  and  more  than  one 
tenth  of  all  Seed-plants.  Not  only  is  it  the  greatest  family, 
but  it  is  the  youngest.  Composites  are  distributed  every- 
where, but  are  most  numerous  in  temperate  regions,  and 
are  mostly  herbs. 


GLOSSARY 


[The  definitions  of  a  glossary  are  often  unsatisfactory.  It  is  much  better  to  con- 
sult the  fuller  explanations  of  the  text  by  means  of  the  index.  The  following  glos- 
sary includes  only  frequently  recurring  technical  terms.  Those  which  are  found  only 
in  reasonably  close  association  with  their  explanation  are  omitted.] 

Akene  :  a  one-seeded  fruit  which  ripens  dry  and  seed-like. 

Alternation  of  generations:  the  alternation  of  gametophyte  and 
sporophyte  in  a  life  history. 

Anemophilous  :  applied  to  flowers  or  plants  which  use  the  wind  as  agent 
of  pollination. 

Anther:  the  sporangium-bearing  part  of  a  stamen. 

Antheridium  :  the  male  organ,  producing  sperms. 

Apetalous  :  applied  to  a  flower  with  no  petals. 

Apocarpous:  applied  to  a  flower  whose  carpels  are  free  from  one  an- 
other. 

Archegonium:  the  female,  egg-producing  organ  of  Bryophytes,  Pteri- 

dophytes.  and  Gymnosperms. 
AscocARP :  a  special  case  containing  asci. 
Ascospore  :  a^spore  formed  within  an  ascus. 

Ascus:  a  delicate  sac  (mother-cell)  within  which  ascospores  develop. 
Asexual  spore  :  one  produced  usually  by  cell-division,  at  least  not  by 

cell-union. 

Calyx  :  the  outer  set  of  floral  leaves. 

Capsule:  in  Bryophytes  the  spore-vessel ;  in  Angiosperms  a  dry  fruit 

which  opens  to  discharge  its  seeds. 
Carpel  :  the  megasporophyll  of  Spermatophytes. 
Chlorophyll  :  the  green  coloring  matter  of  plants. 
Chloroplast  :  the  protoplasmic  body  within  the  cell  which  is  stained 

green  by  chlorophyll. 
Conjugation  :  the  union  of  similar  gametes. 
Corolla  :  the  inner  set  of  floral  leaves. 


384  PLANT   STUDIES 

Cotyledon  :  the  first  leaf  developed  by  an  embryo  sporophyte. 

Cyclic  :  applied  to  an  arrangement  of  leaves  or  floral  parts  in  which 

two  or  more  appear  upon  the  axis  at  the  same  level,  forming  a 

cycle,  or  whorl,  or  verticil. 

Dehiscence  :  the  opening  of  an  organ  to  discharge  its  contents,  as  in 
sporangia,  pollen-sacs,  capsules,  etc. 

DicHOTOMOUS :  applied  to  a  style  of  branching  in  which  the  tip  of  the 
axis  forks. 

Dkecious  :  applied  to  plants  in  which  the  two  sex-organs  are  upon  dif- 
ferent individuals. 

DoRSivENTRAL :  applied  to  a  body  whose  two  surfaces  are  differently 
exposed,  as  an  ordinary  thallus  or  loaf. 

Egg  :  the  female  gamete. 

Embryo  :  a  plant  in  the  earliest  stages  of  its  development  from  the 

spore. 
Embryo-sac  :  the  megaspore  of  Spermatophytes,  which  later  contains 

the  embryo. 
Endosperm  :  the  nourishing  tissue  developed  within  the  embryo-sac, 

and  thought  to  represent  the  female  gametophyte. 
Entomophilous  :  applied  to  flowers  or  plants  which  use  insects  as  agents 

of  pollination. 
Epigynous  :  applied  to  a  flower  whose  outer  parts  appear  to  arise  from 

the  top  of  the  ovary. 

Fertilization  :  the  union  of  sperm  and  egg. 

Filament  :  the  stalk-like  part  of  a  stamen. 

Foot  :  in  Bryophytes  the  part  of  the  sporogoniura  imbedded  in  the 

gametophore ;  in  Pteridophytes  an  organ  of  the  spor(jphyte  embryo 

to  absorb  from  the  gametophyte. 

Gametangium  :  the  organ  within  which  gametes  are  produced. 
Gamete  :  a  sexual  cell,  which  by  union  with  another  produces  a  sexual 

spore. 
Gametophyte  :   in  alternation  of  generations,  the   generation   which 

bears  the  sex  organs. 

Heterogamous  :  applied  to  plants  whose  pairing  gametes  are  unlike. 

Heterosporous  :  applied  to  those  higher  plants  whose  sporophyte  pro- 
duces two  forms  of  asexual  spores. 

Homosporous  :  applied  to  those  plants  whose  sporophyte  produces  simi- 
lar asexual  spores. 


GLOSSARY  385 

Host  :  a  plant  or  animal  attacked  by  a  parasite. 

Hypha  :  an  individual  filament  of  a  mycelium. 

Hypocotyl  :  the  axis  of  the  embryo  sporophyte  between  the  root-tip 

and  the  cotyledons. 
Hypogynous  :  applied  to  a  flower  whose  outer  parts  arise  from  beneath 

the  ovary. 

Inflorescence  :  a  flower-cluster. 

Integument  :  in  Spermatophytes  a  membrane  investing  the  nucellus. 

IsoGAMOUs:  applied  to  plants  whose  pairing  gametes  are  similar. 

Male  cell  :  in  Spermatophytes  the  fertilizing  cell  conducted  by  the 

pollen-tube  to  the  egg. 
Megasporangium  :  a  sporangium  which  produces  only  megaspores. 
Megaspore  :  in  heterosporous  plants  the  large  spore  which  produces  a 

female  gametophyte. 
Megasporophyll  :  a  sporophyll  which  produces  only  megasporangia. 
Mesophyll:  the  tissue  of  a  leaf  between  the  two  epidermal  layers  which 

usually  contains  chloroplasts. 
Microsporangium  :  a  sporangium  which  produces  only  microspores. 
Microspore  :  in  heterosporous  plants  the  small  spore  which  produces  a 

male  gametophyte. 
MiCROSPOROPHYLL :  a  sporophyll  which  produces  only  microsporangia. 
MiCROPYLE :  the  passageway  to  the  nucellus  left  by  the  integument. 
Monoecious  :  applied  to  plants  in  which  the  two  sex  organs  are  upon 

the  same  individual. 
Mycelium  :  the  mat  of  filaments  which  composes  the  working  body  of 

a  fungus. 

Naked  flower  :  one  with  no  floral  leaves. 
Nucellus  :  the  main  body  of  the  ovule. 

Oogonium:  the  female,  egg-producing  organ  of  Thallophytes. 

Oosphere:  the  female  gamete,  or  egg. 

Oospore  :  the  sexual  spore  resulting  from  fertilization. 

Ovary  :  in  Angiosperms  the  bulbous  part  of  the  pistil,  which  contains 

the  ovules. 
Ovule  :  the  megasporangium  of  Spermatophytes. 

Parasite:  a  plant  which  obtains  food  by  attacking  living  plants  or 

animals. 
Perianth  :  the  set  of  floral  leaves  when  not  differentiated  into  calyx 

and  corolla. 


386  PLANT   STUDIES 

Petal:  one  of  the  floral  leaves  which  make  up  the  corolla. 

Photosynthesis  :  the  process  by  which  chloroplasts,  aided  by  light, 
manufacture  carbohydrates  from  carbon  dioxide  and  water. 

Pistil  :  the  central  organ  of  the  flower,  composed  of  one  or  more  car- 
pels. 

Pistillate  :  applied  to  flowers  with  carpels  but  no  stamens. 

Pollen  :  the  microspores  of  Spermatophytes. 

Pollen-tube  :  the  tube  developed  from  the  wall  of  the  pollen  grain 
which  penetrates  to  the  egg  and  conducts  the  male  cells. 

Pollination  :  the  transfer  of  pollen  from  anther  to  ovule  (in  Gyrano- 
sperms)  or  stigma  (in  Angiosperms). 

Polypetalous  :  applied  to  flowers  whose  petals  are  free  from  one  an- 
other. 

Prothallium  :  the  gametophyte  of  Ferns. 

Protonema  :  the  thallus  portion  of  the  gametophyte  of  Mosses. 

Receptacle  :  in  Angiosperms  that  part  of  the  stem  which  is  more  or 
less  modified  to  support  the  parts  of  the  flower. 

Rhizoid  :  a  hair-like  process  developed  by  the  lower  plants  and  by  in- 
dependent gametophytes  to  act  as  a  holdfast  or  absorbing  organ, 
or  both. 

Saprophyte  :  a  plant  which  obtains  food  from  the  dead  bodies  or  body 
products  of  plants  or  animals. 

Scale  :  a  leaf  without  chlorophyll,  and  usually  reduced  in  size. 

Sepal  :  one  of  the  floral  leaves  which  make  up  the  calyx. 

Sexual  spore  :  one  produced  by  the  union  of  gametes. 

Sperm  :  the  male  gamete. 

Spiral  :  applied  to  an  arrangement  of  leaves  or  floral  parts  in  which 
no  two  appear  upon  the  axis  at  the  same  level ;  often  called  alter- 
nate. 

Sporangium  :  the  organ  within  which  asexual  spores  are  produced 
(except  in  Bryophytes). 

Spore  :  a  cell  set  apart  for  reproduction. 

Sporogonium  :  the  leafless  sporophyte  of  Bryophytes. 

Sporophore  :  a  special  branch  bearing  asexual  spores. 

Sporophyll  :  a  leaf  set  apart  to  produce  sporangia. 

Sporophyte  :  in  alternation  of  generations,  the  generation  which  pro- 
duces the  asexual  spores. 

Stamen:  the  microsporophyll  of  Spermatophytes. 

Staminate  :  applied  to  a  flower  with  stamens  but  no  carpels. 

Stigma  :  in  Angiosperms  that  portion  of  the  carpel  (usually  of  the  style) 
prepared  to  receive  pollen. 


GLOSSARY 


387 


Stoma  (pi.  Stomata):  an  epidermal  organ  for  regulating  the  communi- 
cation between  green  tissue  and  the  air. 

Strobilus  :  a  cone-like  cluster  of  sporophylls. 

Style:  the  stalk-like  prolongation  from  the  ovary  which  bears  the 
stigma. 

Symbiont:  an  organism  which  enters  into  the  condition  of  symbiosis. 

Syjibiosis:  usually  applied  to  the  condition  in  which  two  different 
organisms  live  together  in  intimate  and  mutually  helpful  relations. 

Sympetalous  :  applied  to  a  flower  whose  petals  have  coalesced. 

Syncarpous  :  applied  to  a  flower  whose  carpels  have  coalesced. 

Zoospore  :  a  motile  asexual  spore. 

Zygote  :  the  sexual  spore  resulting  from  conjugation. 


Of 


'^K 


INDEX 


Adaptation,  147. 

JEcidiomycetes,  278. 

Alga?,  224.  225. 

Alternation    of  generations,   300, 

321. 
Aneraophilous,  352. 
Angiosperms,  358,  370. 
Animals,  145. 
Anther.  360. 
Antheridiiim,  231,  304. 
Anthoceros.  315. 
Ant  plants.  162. 
Araucaria,  74. 
Archegonium,  305. 
Ascocarp,  274. 
Ascoraycetes,  273. 
Ascospore,  275. 
Ascus,  275. 
Asexual  spore,  229. 
Assimilation,  154. 
Associations,  1,  169. 

Bacteria,  291. 
Banyan,  105. 
Basidiomycetes,  284. 
Begonia,  25. 
Birch,  71. 
Body,  2,  222,  226. 
Botrychium,  244. 
Branched  leaves,  19. 
Bryophytes,  222,  299,  320,  344. 


Bud,  73,  141. 
Bulb,  75. 
Burdock,  121. 

Calyx,  79. 
Capsule,  303. 
Carbohydrate,  153. 
Carnivorous  plants,  164, 
Carpel,  79,  350,  362. 
Carrot,  120. 
Cell,  226. 
Characea^,  262. 
Chlorophycea?,  236. 
Chlorophyll,  149. 
Chloroplast,  39,  152,  228. 
Chrysanthemum,  23.' 
Cilia,  230. 
Cladophora,  241. 
Cleistogaray,  130. 
Club-mosses,  340. 
Cocklebur,  120. 
Compass  plant,  10.  48.  193. 
Compound  leaves,  19. 
Conifer,  83,  350. 
Conjugation,  237. 
Corolla,  79. 
Cortex,  83. 
Cottonwood,  70. 
Cotyledon,  51,  73,  369. 
Cyanophycea>,  232. 
Cycad,  22,  354. 

389 


390 


PLANT   STUDIES 


Cyclic,  365,  366,  377,  379. 
Cypress,  96. 
Cytoplasm,  227. 

Dandelion,  114. 
Desmids,  248. 
Diatoms,  261. 
Dichotomous,  251. 
Dicotyledon,  83,  305,  376. 
Digestion,  154. 
Dionaea,  168. 
Dodder,  106. 
Drosera,  166. 

Ecological  factors,  170. 
Ecology,  4. 
Edogonium,  238. 
Egg,  110,  231. 
Elm,  67,  68. 
Embryo,  111,  352,  369. 
Embryo-sac,  350. 
Endosperm,  351. 
Entomophilous,  359. 
Epidermis,  37,  83. 
Epigynous,  365. 
Equisetum,  337. 
Evolution,  223. 

Fern,  55,  56,  85,  334. 
Fertilization,  351,  368. 
Filament,  360. 
Flower,  76,  140,  364 ;  and  insects, 

123,  162. 
Foliage,  6,  28,  35. 
Foot,  303. 
Fruit,  368. 
Fucus,  251. 
Fungi,  224,  264. 

Gametangium,  231. 
Gamete,  230. 
Gametophore,  303. 


Gametophyte,  303,  323,  351,  366, 

367,  375. 
Germination,   111,   138;    of  seed, 

369. 
Geotropism,  69,  91,  138. 
Glceocapsa,  232. 
Gymnosperms,  345,  358. 

Hair,  136,  198. 
Haustoria,  266. 
Heliotropism,  12,  68,  139. 
Heterogamous,  231. 
Heterospory,  330. 
Homospory,  332. 
Horsetails,  337. 
Host,  264. 

Hydrophytes,  175,  177. 
Hydrotropism,  91,  138. 
HyphiB,  265. 
Hypogynous,  365. 

Insects  and  flowers,  123,  162. 
Integument,  350. 
Isogamous,  231. 

Jungermannia,  314. 

Lady-slipper,  132-136. 
Laminaria,  249. 
Latex,  136. 
Leaves,  28,  35,  139. 
Lichens,  159,  293. 
Life-relations,  4. 
Light,  143,  174. 
Light-relations,  8. 
Linden,  116. 
Liverworts,  308. 
Lycopodium,  340. 

Maple,  26,  115. 
Marchantia,  107,  309. 


INDEX 


391 


Megasporangia,  332. 
Megaspore,  332. 
Megasporophyll,  349,  362. 
Mesophyll,  39. 
Mesophytes,  175,  214. 
Micropyle,  350. 
Microsporangia,  332. 
Microspores,  332. 
Microsporophyll,  346,  359. 
Migration,  147. 
Mildews,  273. 
Monocotyledon,  85,  365. 
Mosaic,  24. 
Mosses,  316. 
Motile  leaves,  10,  193. 
Moulds,  276. 
Mucor,  268. 
Mushroom,  285. 
Mycelium,  265. 
Mycorhiza,  159. 

Naked  flower,  364. 
Nectar,  123. 
Nostoc,  233. 
Nucellus,  350. 
Nucleus,  227. 
Nutrition,  3,  149,  223. 

Oak,  69. 

CEdogonium,  238. 
Oogonium,  231. 
Oospore,  240. 
Orchid,  98. 
Oscillaria,  234. 
Ovary,  125,  362. 
Ovule,  350. 

Palm,  86. 

Parasite,  106,  150,  157. 

Peronspora,  271. 

Petal,  79. 

Petiole,  35. 


PhsBophyceae,  248. 

Photosynthesis,  28,  150. 

Phycomycetes,  267. 

Physiology,  149. 

Pine,  65,  66. 

Pistil,  77,  350,  363. 

Pitcher  plants,  165. 

Pith,  83. 

Plastid,  228. 

Pleurococcus,  236. 

Pollen,  77,  346  ;  tube,  351. 

Pollination,  77,  121,  123. 

Potato,  76. 

Protandry,  128. 

Protection,  41,  137,  189. 

Proteid,  153. 

Prothallium,  322. 

Protogyny,  128. 

Protoneraa,  303. 

Protoplasm,  227. 

Pteridophytes,  222,  320,  343,  344. 

Rain,  51. 

Raspberry,  91. 

Receptacle,  81. 

Redbud,  10. 

Reproduction,  3,  109,  223,  228. 

Respiration,  32,  154. 

Rhizoids,  308. 

Rhodophycea?,  254. 

Rivalry,  146. 

Root,  89,  107,  138. 

Rootstock,  75. 

Root  tubercles,  161. 

Rosette  habit,  17,  47. 

Rubber  tree,  104. 

Saprolegnia,  267. 
Saprophyte,  150,  157. 
Sargassum,  251. 
Seed,  352 ;  dispersal,  79, 112. 


392 


PLANT   STUDIES 


Selaginella,  340. 

Sensitive  plants,  11,  50. 

Sepal,  79. 

Seta,  303. 

Sexual  spore,  230. 

Shoot,  53. 

Slime  moulds,  290. 

Smilax,  61. 

Soil,  90,  145,  173. 

Sperm,  231. 

Spermatophytes,  222,  343,  344. 

Sphagnum,  318. 

Spiral,  365. 

Spirogyra,  244. 

Sporangium,  230,  325. 

Spore,  110,  229. 

Sporogonium,  303,  306. 

Sporophore,  266. 

Sporophyll,  346. 

Sporophyte,  303,  325. 

Stamen,  79,  346,  359. 

Stem,  54,  83,  139. 

Stigma,  125,  362. 

Stipules,  35. 

Stomata,  38. 

Strobilus,  338. 

Struggle  for  existence,  142. 

Style,  125,  362. 

Symbionts,  158,  295. 

Symbiosis,  158,  295. 


Temperature,  145,  171. 
Thallophytes,  222,  224,  299,  344. 
Transpiration,  31,  154. 
Tuber,  74. 
Tumbleweed,  117. 

Ulothrix,  237. 

Vascular  system,  83. 

Vaucheria,  242. 

Vegetative     multiplication,     109, 

229. 
Veins,  36,  40. 
Violet,  117. 

Water,   142,   151,  170;  reservoirs, 

201. 
Water  ferns,  336. 
Wheat  rust,  279. 
Wind,  174. 
Witch  hazel,  118. 
Woodbine,  63. 

Xerophytes,  175,  188. 

Yeast,  278. 
Yucca,  131. 

Zoospore,  230. 
Zygotes,  237. 


THE   END 


t^m^^  (13) 


A  USEFUL  AND  ATTRACTIVE  TEXT 


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his  everyday  observation.  Without  sacrificing  the  essentially 
scientific  nature  of  the  subject  the  author  recognizes  it  as  a 
science  which  has  a  very  definite  bearing  upon  everyday  life. 

The  author  believes  that  the  class-room  work  should  be 
accompanied  by  suitable  laboratory  experimentation  by  the 
pupils  supplemented  with  demonstrative  experiments  by  the 
instructor. 

Stress  is  laid  upon  the  beneficial  results  to  which  the  study 
of  Physics  has  led  as  its  development  has  progressed.  Espe- 
cial attention  has  been  given  to  the  interesting  historical 
development  of  the  subject.  Portraits  and  adequate  bio- 
graphical sketches  of  many  scientists  to  whom  the  discovery 
of  great  principles  is  due  have  been  inserted. 

The  problems  throughout  the  book  eliminate  the  usual 
exercises  in  pure  reduction  and  substitute  those  of  a  more 
concrete  and  practical  nature. 

The  apparatus  described  is  as  simple  as  experience  has 
shown  to  be  consistent  with  satisfactory  results. 

The  illustrations  are  abundant  and  each  is  given  a  descrip- 
tive legend. 

To  aid  the  pupil  in  reviewing  and  the  teacher  in  quizzing, 
there  are  summaries  at  the  ends  of  the  chapters. 

No  subject  has  been  left  out  that  is  called  for  in  the  report 
of  the  College  Entrance  Requirement  Board. 

D.     APPLETON     AND     COMPANY 

NEW  YORK  CHICAGO 

454e 


TWENTIETH  CENTURY  TEXT-BOOKS. 


TEXT-BOOKS  OF  ZOOLOGY, 

By  David  Starr  Jordan,  President  of  Leland  Stan- 
ford Jr.  University;  Vernon  Lyman  Kellogg,  Professor 
of  Entomology;  Harold  Heath,  Assistant  Professor  of 
Invertebrate  Zoology. 

Evolution  and  Animal  Life. 

This  is  a  popular  discussion  of  the  facts,  processes,  laws,  and  theories 
relating  to  the  life  and  evolution  of  animals.  The  reader  of  it  will  have 
a  very  clear  idea  of  the  all-important  theory  of  evolution  as  it  has  been 
developed  and  as  it  is  held  to-day  by  scientists.  8vo.  Cloth,  with  about 
300  illustrations,  I2.50  net  ;  postage  20  cents  additional. 

Animal  Studies. 

A  compact  but  complete  treatment  of  elementary  zoology,  especially 
prepared  for  institutions  of  learning  that  prefer  to  find  in  a  single  book 
an  ecological  as  well  as  morphological  sui-vey  of  the  animal  world,  i2mo. 
Cloth,  I1.25  net. 

Animal  Life. 

An  elementary  account  of  animal  ecology — that  is,  of  the  relations 
of  animals  to  their  surroundings.  It  treats  of  animals  from  the  stand- 
poiut  of  the  observer,  and  shows  why  the  present  conditions  and  habits 
of  animal  life  are  as  we  find  them,     i2mo.     Cloth,  f  1.20  net. 

Animal  Forms. 

This  book  deals  in  an  elementary  way  with  animal  morphology.  It 
describes  the  structure  and  life  processes  of  animals,  from  the  lowest 
creations  to  the  highest  and  most  complex.     i2mo.     Cloth,  $1.10  net. 

Animals. 

This  consists  of  "Animal  Life"  and  "Animal  Forms"  bound  in  one 
volume.     i2mo.     Cloth,  $1.80. 

Animal  Structures. 

A  laboratory  guide  in  the  teaching  of  elementary  zoology.  l2mo. 
Cloth,  50  cents  net. 

D.     APPLETON     AND     COMPANY, 

NEW  YORK.  BOSTON.  CHICAGO.  LONDON. 


OLDEST   OF   THE  ARTS,  NEWEST   OF   THE 
SQENCES* 


Practical  Agriculture. 

By  Charles  C.  James,  M.  A.,  Deputy  Minister  of 
Agriculture  for  Ontario,  formerly  Professor  of  Chemistry 
at  the  Ontario  Agricultural  College.  American  Edition, 
edited  by  John  Craig,  Professor  of  Horticulture  in  the 
Iowa  Agricultural  College.  With  numerous  Illustrations. 
i2mo.     Cloth,  80  cents. 

This  excellent  book  shows  how  easy,  interesting,  and  prac- 
tical the  teaching  of  agriculture  in  common  schools  really  is.  It 
imparts  a  knowledge  of  the  science  of  Agriculture  as  distinct 
from  the  art — that  is,  a  knowledge  of  the  why  rather  than  of  the 
how.  This  science  consists  of  a  mingling  of  chemistry,  geology, 
botany,  entomology,  physiology,  bacteriology,  etc.  The  founda- 
tion principles  of  these  subjects  have  been  included  and  their 
applications  clearly  and  suggestively  shown. 

Professor  James  gives  his  subject  the  broadest  interpretation. 
Agriculture  is  for  him  the  cultivation  of  the  soil  for  food  products 
and  any  other  useful  growths  of  the  field  or  garden.  It  includes 
tillage,  husbandry-,  farming  in  general,  and  any  industry  practised 
by  a  cultivator  of  the  soil,  as  breeding,  rearing,  dairying,  etc. 

Governor  JAMES  A.  MOUNT,  Indianapolis,  Ind. : 

"  I  would  that  such  works  were  in  every  farm  home.  They  would  give 
the  farmer  a  broader  view  of  his  vocation.  He  would  view  it  as  an  art,  a 
science,  a  profession,  and  not  as  mere  drudgery,  requiring  manual  labor 
instead  of  mental  activity." 

A.  W.   RANKIN,   Inspector    State  Graded  Schools,  Minneapolis: 

"  I  think  James's  '  Practical  Agriculture'  is  the  best  book  I  have  seen  on 
this  subject.  I  heartily  approve  of  its  purpose,  and  shall  urge  its  use  wher- 
ever an  opportunity  offers. " 


APPLETON     AND     COMPANY,     NEW     YORK 


e^: 


Carolina  State  University  Libraries 

QK47.C86  1911 

PLANT  STUDIES  AN  ELEMENTARY  BOTANY 


S02776875   R 


